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| author | Daniel Baumann <daniel.baumann@progress-linux.org> | 2024-04-07 18:49:45 +0000 |
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| committer | Daniel Baumann <daniel.baumann@progress-linux.org> | 2024-04-07 18:49:45 +0000 |
| commit | 2c3c1048746a4622d8c89a29670120dc8fab93c4 (patch) | |
| tree | 848558de17fb3008cdf4d861b01ac7781903ce39 /Documentation/trace/histogram-design.rst | |
| parent | Initial commit. (diff) | |
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Adding upstream version 6.1.76.upstream/6.1.76
Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>
Diffstat (limited to 'Documentation/trace/histogram-design.rst')
| -rw-r--r-- | Documentation/trace/histogram-design.rst | 2115 |
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diff --git a/Documentation/trace/histogram-design.rst b/Documentation/trace/histogram-design.rst new file mode 100644 index 000000000..088c8cce7 --- /dev/null +++ b/Documentation/trace/histogram-design.rst @@ -0,0 +1,2115 @@ +.. SPDX-License-Identifier: GPL-2.0 + +====================== +Histogram Design Notes +====================== + +:Author: Tom Zanussi <zanussi@kernel.org> + +This document attempts to provide a description of how the ftrace +histograms work and how the individual pieces map to the data +structures used to implement them in trace_events_hist.c and +tracing_map.c. + +Note: All the ftrace histogram command examples assume the working + directory is the ftrace /tracing directory. For example:: + + # cd /sys/kernel/debug/tracing + +Also, the histogram output displayed for those commands will be +generally be truncated - only enough to make the point is displayed. + +'hist_debug' trace event files +============================== + +If the kernel is compiled with CONFIG_HIST_TRIGGERS_DEBUG set, an +event file named 'hist_debug' will appear in each event's +subdirectory. This file can be read at any time and will display some +of the hist trigger internals described in this document. Specific +examples and output will be described in test cases below. + +Basic histograms +================ + +First, basic histograms. Below is pretty much the simplest thing you +can do with histograms - create one with a single key on a single +event and cat the output:: + + # echo 'hist:keys=pid' >> events/sched/sched_waking/trigger + + # cat events/sched/sched_waking/hist + + { pid: 18249 } hitcount: 1 + { pid: 13399 } hitcount: 1 + { pid: 17973 } hitcount: 1 + { pid: 12572 } hitcount: 1 + ... + { pid: 10 } hitcount: 921 + { pid: 18255 } hitcount: 1444 + { pid: 25526 } hitcount: 2055 + { pid: 5257 } hitcount: 2055 + { pid: 27367 } hitcount: 2055 + { pid: 1728 } hitcount: 2161 + + Totals: + Hits: 21305 + Entries: 183 + Dropped: 0 + +What this does is create a histogram on the sched_waking event using +pid as a key and with a single value, hitcount, which even if not +explicitly specified, exists for every histogram regardless. + +The hitcount value is a per-bucket value that's automatically +incremented on every hit for the given key, which in this case is the +pid. + +So in this histogram, there's a separate bucket for each pid, and each +bucket contains a value for that bucket, counting the number of times +sched_waking was called for that pid. + +Each histogram is represented by a hist_data struct. + +To keep track of each key and value field in the histogram, hist_data +keeps an array of these fields named fields[]. The fields[] array is +an array containing struct hist_field representations of each +histogram val and key in the histogram (variables are also included +here, but are discussed later). So for the above histogram we have one +key and one value; in this case the one value is the hitcount value, +which all histograms have, regardless of whether they define that +value or not, which the above histogram does not. + +Each struct hist_field contains a pointer to the ftrace_event_field +from the event's trace_event_file along with various bits related to +that such as the size, offset, type, and a hist_field_fn_t function, +which is used to grab the field's data from the ftrace event buffer +(in most cases - some hist_fields such as hitcount don't directly map +to an event field in the trace buffer - in these cases the function +implementation gets its value from somewhere else). The flags field +indicates which type of field it is - key, value, variable, variable +reference, etc., with value being the default. + +The other important hist_data data structure in addition to the +fields[] array is the tracing_map instance created for the histogram, +which is held in the .map member. The tracing_map implements the +lock-free hash table used to implement histograms (see +kernel/trace/tracing_map.h for much more discussion about the +low-level data structures implementing the tracing_map). For the +purposes of this discussion, the tracing_map contains a number of +buckets, each bucket corresponding to a particular tracing_map_elt +object hashed by a given histogram key. + +Below is a diagram the first part of which describes the hist_data and +associated key and value fields for the histogram described above. As +you can see, there are two fields in the fields array, one val field +for the hitcount and one key field for the pid key. + +Below that is a diagram of a run-time snapshot of what the tracing_map +might look like for a given run. It attempts to show the +relationships between the hist_data fields and the tracing_map +elements for a couple hypothetical keys and values.:: + + +------------------+ + | hist_data | + +------------------+ +----------------+ + | .fields[] |---->| val = hitcount |----------------------------+ + +----------------+ +----------------+ | + | .map | | .size | | + +----------------+ +--------------+ | + | .offset | | + +--------------+ | + | .fn() | | + +--------------+ | + . | + . | + . | + +----------------+ <--- n_vals | + | key = pid |----------------------------|--+ + +----------------+ | | + | .size | | | + +--------------+ | | + | .offset | | | + +--------------+ | | + | .fn() | | | + +----------------+ <--- n_fields | | + | unused | | | + +----------------+ | | + | | | | + +--------------+ | | + | | | | + +--------------+ | | + | | | | + +--------------+ | | + n_keys = n_fields - n_vals | | + +The hist_data n_vals and n_fields delineate the extent of the fields[] | | +array and separate keys from values for the rest of the code. | | + +Below is a run-time representation of the tracing_map part of the | | +histogram, with pointers from various parts of the fields[] array | | +to corresponding parts of the tracing_map. | | + +The tracing_map consists of an array of tracing_map_entrys and a set | | +of preallocated tracing_map_elts (abbreviated below as map_entry and | | +map_elt). The total number of map_entrys in the hist_data.map array = | | +map->max_elts (actually map->map_size but only max_elts of those are | | +used. This is a property required by the map_insert() algorithm). | | + +If a map_entry is unused, meaning no key has yet hashed into it, its | | +.key value is 0 and its .val pointer is NULL. Once a map_entry has | | +been claimed, the .key value contains the key's hash value and the | | +.val member points to a map_elt containing the full key and an entry | | +for each key or value in the map_elt.fields[] array. There is an | | +entry in the map_elt.fields[] array corresponding to each hist_field | | +in the histogram, and this is where the continually aggregated sums | | +corresponding to each histogram value are kept. | | + +The diagram attempts to show the relationship between the | | +hist_data.fields[] and the map_elt.fields[] with the links drawn | | +between diagrams:: + + +-----------+ | | + | hist_data | | | + +-----------+ | | + | .fields | | | + +---------+ +-----------+ | | + | .map |---->| map_entry | | | + +---------+ +-----------+ | | + | .key |---> 0 | | + +---------+ | | + | .val |---> NULL | | + +-----------+ | | + | map_entry | | | + +-----------+ | | + | .key |---> pid = 999 | | + +---------+ +-----------+ | | + | .val |--->| map_elt | | | + +---------+ +-----------+ | | + . | .key |---> full key * | | + . +---------+ +---------------+ | | + . | .fields |--->| .sum (val) |<-+ | + +-----------+ +---------+ | 2345 | | | + | map_entry | +---------------+ | | + +-----------+ | .offset (key) |<----+ + | .key |---> 0 | 0 | | | + +---------+ +---------------+ | | + | .val |---> NULL . | | + +-----------+ . | | + | map_entry | . | | + +-----------+ +---------------+ | | + | .key | | .sum (val) or | | | + +---------+ +---------+ | .offset (key) | | | + | .val |--->| map_elt | +---------------+ | | + +-----------+ +---------+ | .sum (val) or | | | + | map_entry | | .offset (key) | | | + +-----------+ +---------------+ | | + | .key |---> pid = 4444 | | + +---------+ +-----------+ | | + | .val | | map_elt | | | + +---------+ +-----------+ | | + | .key |---> full key * | | + +---------+ +---------------+ | | + | .fields |--->| .sum (val) |<-+ | + +---------+ | 65523 | | + +---------------+ | + | .offset (key) |<----+ + | 0 | + +---------------+ + . + . + . + +---------------+ + | .sum (val) or | + | .offset (key) | + +---------------+ + | .sum (val) or | + | .offset (key) | + +---------------+ + +Abbreviations used in the diagrams:: + + hist_data = struct hist_trigger_data + hist_data.fields = struct hist_field + fn = hist_field_fn_t + map_entry = struct tracing_map_entry + map_elt = struct tracing_map_elt + map_elt.fields = struct tracing_map_field + +Whenever a new event occurs and it has a hist trigger associated with +it, event_hist_trigger() is called. event_hist_trigger() first deals +with the key: for each subkey in the key (in the above example, there +is just one subkey corresponding to pid), the hist_field that +represents that subkey is retrieved from hist_data.fields[] and the +hist_field_fn_t fn() associated with that field, along with the +field's size and offset, is used to grab that subkey's data from the +current trace record. + +Once the complete key has been retrieved, it's used to look that key +up in the tracing_map. If there's no tracing_map_elt associated with +that key, an empty one is claimed and inserted in the map for the new +key. In either case, the tracing_map_elt associated with that key is +returned. + +Once a tracing_map_elt available, hist_trigger_elt_update() is called. +As the name implies, this updates the element, which basically means +updating the element's fields. There's a tracing_map_field associated +with each key and value in the histogram, and each of these correspond +to the key and value hist_fields created when the histogram was +created. hist_trigger_elt_update() goes through each value hist_field +and, as for the keys, uses the hist_field's fn() and size and offset +to grab the field's value from the current trace record. Once it has +that value, it simply adds that value to that field's +continually-updated tracing_map_field.sum member. Some hist_field +fn()s, such as for the hitcount, don't actually grab anything from the +trace record (the hitcount fn() just increments the counter sum by 1), +but the idea is the same. + +Once all the values have been updated, hist_trigger_elt_update() is +done and returns. Note that there are also tracing_map_fields for +each subkey in the key, but hist_trigger_elt_update() doesn't look at +them or update anything - those exist only for sorting, which can +happen later. + +Basic histogram test +-------------------- + +This is a good example to try. It produces 3 value fields and 2 key +fields in the output:: + + # echo 'hist:keys=common_pid,call_site.sym:values=bytes_req,bytes_alloc,hitcount' >> events/kmem/kmalloc/trigger + +To see the debug data, cat the kmem/kmalloc's 'hist_debug' file. It +will show the trigger info of the histogram it corresponds to, along +with the address of the hist_data associated with the histogram, which +will become useful in later examples. It then displays the number of +total hist_fields associated with the histogram along with a count of +how many of those correspond to keys and how many correspond to values. + +It then goes on to display details for each field, including the +field's flags and the position of each field in the hist_data's +fields[] array, which is useful information for verifying that things +internally appear correct or not, and which again will become even +more useful in further examples:: + + # cat events/kmem/kmalloc/hist_debug + + # event histogram + # + # trigger info: hist:keys=common_pid,call_site.sym:vals=hitcount,bytes_req,bytes_alloc:sort=hitcount:size=2048 [active] + # + + hist_data: 000000005e48c9a5 + + n_vals: 3 + n_keys: 2 + n_fields: 5 + + val fields: + + hist_data->fields[0]: + flags: + VAL: HIST_FIELD_FL_HITCOUNT + type: u64 + size: 8 + is_signed: 0 + + hist_data->fields[1]: + flags: + VAL: normal u64 value + ftrace_event_field name: bytes_req + type: size_t + size: 8 + is_signed: 0 + + hist_data->fields[2]: + flags: + VAL: normal u64 value + ftrace_event_field name: bytes_alloc + type: size_t + size: 8 + is_signed: 0 + + key fields: + + hist_data->fields[3]: + flags: + HIST_FIELD_FL_KEY + ftrace_event_field name: common_pid + type: int + size: 8 + is_signed: 1 + + hist_data->fields[4]: + flags: + HIST_FIELD_FL_KEY + ftrace_event_field name: call_site + type: unsigned long + size: 8 + is_signed: 0 + +The commands below can be used to clean things up for the next test:: + + # echo '!hist:keys=common_pid,call_site.sym:values=bytes_req,bytes_alloc,hitcount' >> events/kmem/kmalloc/trigger + +Variables +========= + +Variables allow data from one hist trigger to be saved by one hist +trigger and retrieved by another hist trigger. For example, a trigger +on the sched_waking event can capture a timestamp for a particular +pid, and later a sched_switch event that switches to that pid event +can grab the timestamp and use it to calculate a time delta between +the two events:: + + # echo 'hist:keys=pid:ts0=common_timestamp.usecs' >> + events/sched/sched_waking/trigger + + # echo 'hist:keys=next_pid:wakeup_lat=common_timestamp.usecs-$ts0' >> + events/sched/sched_switch/trigger + +In terms of the histogram data structures, variables are implemented +as another type of hist_field and for a given hist trigger are added +to the hist_data.fields[] array just after all the val fields. To +distinguish them from the existing key and val fields, they're given a +new flag type, HIST_FIELD_FL_VAR (abbreviated FL_VAR) and they also +make use of a new .var.idx field member in struct hist_field, which +maps them to an index in a new map_elt.vars[] array added to the +map_elt specifically designed to store and retrieve variable values. +The diagram below shows those new elements and adds a new variable +entry, ts0, corresponding to the ts0 variable in the sched_waking +trigger above. + +sched_waking histogram +----------------------:: + + +------------------+ + | hist_data |<-------------------------------------------------------+ + +------------------+ +-------------------+ | + | .fields[] |-->| val = hitcount | | + +----------------+ +-------------------+ | + | .map | | .size | | + +----------------+ +-----------------+ | + | .offset | | + +-----------------+ | + | .fn() | | + +-----------------+ | + | .flags | | + +-----------------+ | + | .var.idx | | + +-------------------+ | + | var = ts0 | | + +-------------------+ | + | .size | | + +-----------------+ | + | .offset | | + +-----------------+ | + | .fn() | | + +-----------------+ | + | .flags & FL_VAR | | + +-----------------+ | + | .var.idx |----------------------------+-+ | + +-----------------+ | | | + . | | | + . | | | + . | | | + +-------------------+ <--- n_vals | | | + | key = pid | | | | + +-------------------+ | | | + | .size | | | | + +-----------------+ | | | + | .offset | | | | + +-----------------+ | | | + | .fn() | | | | + +-----------------+ | | | + | .flags & FL_KEY | | | | + +-----------------+ | | | + | .var.idx | | | | + +-------------------+ <--- n_fields | | | + | unused | | | | + +-------------------+ | | | + | | | | | + +-----------------+ | | | + | | | | | + +-----------------+ | | | + | | | | | + +-----------------+ | | | + | | | | | + +-----------------+ | | | + | | | | | + +-----------------+ | | | + n_keys = n_fields - n_vals | | | + | | | + +This is very similar to the basic case. In the above diagram, we can | | | +see a new .flags member has been added to the struct hist_field | | | +struct, and a new entry added to hist_data.fields representing the ts0 | | | +variable. For a normal val hist_field, .flags is just 0 (modulo | | | +modifier flags), but if the value is defined as a variable, the .flags | | | +contains a set FL_VAR bit. | | | + +As you can see, the ts0 entry's .var.idx member contains the index | | | +into the tracing_map_elts' .vars[] array containing variable values. | | | +This idx is used whenever the value of the variable is set or read. | | | +The map_elt.vars idx assigned to the given variable is assigned and | | | +saved in .var.idx by create_tracing_map_fields() after it calls | | | +tracing_map_add_var(). | | | + +Below is a representation of the histogram at run-time, which | | | +populates the map, along with correspondence to the above hist_data and | | | +hist_field data structures. | | | + +The diagram attempts to show the relationship between the | | | +hist_data.fields[] and the map_elt.fields[] and map_elt.vars[] with | | | +the links drawn between diagrams. For each of the map_elts, you can | | | +see that the .fields[] members point to the .sum or .offset of a key | | | +or val and the .vars[] members point to the value of a variable. The | | | +arrows between the two diagrams show the linkages between those | | | +tracing_map members and the field definitions in the corresponding | | | +hist_data fields[] members.:: + + +-----------+ | | | + | hist_data | | | | + +-----------+ | | | + | .fields | | | | + +---------+ +-----------+ | | | + | .map |---->| map_entry | | | | + +---------+ +-----------+ | | | + | .key |---> 0 | | | + +---------+ | | | + | .val |---> NULL | | | + +-----------+ | | | + | map_entry | | | | + +-----------+ | | | + | .key |---> pid = 999 | | | + +---------+ +-----------+ | | | + | .val |--->| map_elt | | | | + +---------+ +-----------+ | | | + . | .key |---> full key * | | | + . +---------+ +---------------+ | | | + . | .fields |--->| .sum (val) | | | | + . +---------+ | 2345 | | | | + . +--| .vars | +---------------+ | | | + . | +---------+ | .offset (key) | | | | + . | | 0 | | | | + . | +---------------+ | | | + . | . | | | + . | . | | | + . | . | | | + . | +---------------+ | | | + . | | .sum (val) or | | | | + . | | .offset (key) | | | | + . | +---------------+ | | | + . | | .sum (val) or | | | | + . | | .offset (key) | | | | + . | +---------------+ | | | + . | | | | + . +---------------->+---------------+ | | | + . | ts0 |<--+ | | + . | 113345679876 | | | | + . +---------------+ | | | + . | unused | | | | + . | | | | | + . +---------------+ | | | + . . | | | + . . | | | + . . | | | + . +---------------+ | | | + . | unused | | | | + . | | | | | + . +---------------+ | | | + . | unused | | | | + . | | | | | + . +---------------+ | | | + . | | | + +-----------+ | | | + | map_entry | | | | + +-----------+ | | | + | .key |---> pid = 4444 | | | + +---------+ +-----------+ | | | + | .val |--->| map_elt | | | | + +---------+ +-----------+ | | | + . | .key |---> full key * | | | + . +---------+ +---------------+ | | | + . | .fields |--->| .sum (val) | | | | + +---------+ | 2345 | | | | + +--| .vars | +---------------+ | | | + | +---------+ | .offset (key) | | | | + | | 0 | | | | + | +---------------+ | | | + | . | | | + | . | | | + | . | | | + | +---------------+ | | | + | | .sum (val) or | | | | + | | .offset (key) | | | | + | +---------------+ | | | + | | .sum (val) or | | | | + | | .offset (key) | | | | + | +---------------+ | | | + | | | | + | +---------------+ | | | + +---------------->| ts0 |<--+ | | + | 213499240729 | | | + +---------------+ | | + | unused | | | + | | | | + +---------------+ | | + . | | + . | | + . | | + +---------------+ | | + | unused | | | + | | | | + +---------------+ | | + | unused | | | + | | | | + +---------------+ | | + +For each used map entry, there's a map_elt pointing to an array of | | +.vars containing the current value of the variables associated with | | +that histogram entry. So in the above, the timestamp associated with | | +pid 999 is 113345679876, and the timestamp variable in the same | | +.var.idx for pid 4444 is 213499240729. | | + +sched_switch histogram | | +---------------------- | | + +The sched_switch histogram paired with the above sched_waking | | +histogram is shown below. The most important aspect of the | | +sched_switch histogram is that it references a variable on the | | +sched_waking histogram above. | | + +The histogram diagram is very similar to the others so far displayed, | | +but it adds variable references. You can see the normal hitcount and | | +key fields along with a new wakeup_lat variable implemented in the | | +same way as the sched_waking ts0 variable, but in addition there's an | | +entry with the new FL_VAR_REF (short for HIST_FIELD_FL_VAR_REF) flag. | | + +Associated with the new var ref field are a couple of new hist_field | | +members, var.hist_data and var_ref_idx. For a variable reference, the | | +var.hist_data goes with the var.idx, which together uniquely identify | | +a particular variable on a particular histogram. The var_ref_idx is | | +just the index into the var_ref_vals[] array that caches the values of | | +each variable whenever a hist trigger is updated. Those resulting | | +values are then finally accessed by other code such as trace action | | +code that uses the var_ref_idx values to assign param values. | | + +The diagram below describes the situation for the sched_switch | | +histogram referred to before:: + + # echo 'hist:keys=next_pid:wakeup_lat=common_timestamp.usecs-$ts0' >> | | + events/sched/sched_switch/trigger | | + | | + +------------------+ | | + | hist_data | | | + +------------------+ +-----------------------+ | | + | .fields[] |-->| val = hitcount | | | + +----------------+ +-----------------------+ | | + | .map | | .size | | | + +----------------+ +---------------------+ | | + +--| .var_refs[] | | .offset | | | + | +----------------+ +---------------------+ | | + | | .fn() | | | + | var_ref_vals[] +---------------------+ | | + | +-------------+ | .flags | | | + | | $ts0 |<---+ +---------------------+ | | + | +-------------+ | | .var.idx | | | + | | | | +---------------------+ | | + | +-------------+ | | .var.hist_data | | | + | | | | +---------------------+ | | + | +-------------+ | | .var_ref_idx | | | + | | | | +-----------------------+ | | + | +-------------+ | | var = wakeup_lat | | | + | . | +-----------------------+ | | + | . | | .size | | | + | . | +---------------------+ | | + | +-------------+ | | .offset | | | + | | | | +---------------------+ | | + | +-------------+ | | .fn() | | | + | | | | +---------------------+ | | + | +-------------+ | | .flags & FL_VAR | | | + | | +---------------------+ | | + | | | .var.idx | | | + | | +---------------------+ | | + | | | .var.hist_data | | | + | | +---------------------+ | | + | | | .var_ref_idx | | | + | | +---------------------+ | | + | | . | | + | | . | | + | | . | | + | | +-----------------------+ <--- n_vals | | + | | | key = pid | | | + | | +-----------------------+ | | + | | | .size | | | + | | +---------------------+ | | + | | | .offset | | | + | | +---------------------+ | | + | | | .fn() | | | + | | +---------------------+ | | + | | | .flags | | | + | | +---------------------+ | | + | | | .var.idx | | | + | | +-----------------------+ <--- n_fields | | + | | | unused | | | + | | +-----------------------+ | | + | | | | | | + | | +---------------------+ | | + | | | | | | + | | +---------------------+ | | + | | | | | | + | | +---------------------+ | | + | | | | | | + | | +---------------------+ | | + | | | | | | + | | +---------------------+ | | + | | n_keys = n_fields - n_vals | | + | | | | + | | | | + | | +-----------------------+ | | + +---------------------->| var_ref = $ts0 | | | + | +-----------------------+ | | + | | .size | | | + | +---------------------+ | | + | | .offset | | | + | +---------------------+ | | + | | .fn() | | | + | +---------------------+ | | + | | .flags & FL_VAR_REF | | | + | +---------------------+ | | + | | .var.idx |--------------------------+ | + | +---------------------+ | + | | .var.hist_data |----------------------------+ + | +---------------------+ + +---| .var_ref_idx | + +---------------------+ + +Abbreviations used in the diagrams:: + + hist_data = struct hist_trigger_data + hist_data.fields = struct hist_field + fn = hist_field_fn_t + FL_KEY = HIST_FIELD_FL_KEY + FL_VAR = HIST_FIELD_FL_VAR + FL_VAR_REF = HIST_FIELD_FL_VAR_REF + +When a hist trigger makes use of a variable, a new hist_field is +created with flag HIST_FIELD_FL_VAR_REF. For a VAR_REF field, the +var.idx and var.hist_data take the same values as the referenced +variable, as well as the referenced variable's size, type, and +is_signed values. The VAR_REF field's .name is set to the name of the +variable it references. If a variable reference was created using the +explicit system.event.$var_ref notation, the hist_field's system and +event_name variables are also set. + +So, in order to handle an event for the sched_switch histogram, +because we have a reference to a variable on another histogram, we +need to resolve all variable references first. This is done via the +resolve_var_refs() calls made from event_hist_trigger(). What this +does is grabs the var_refs[] array from the hist_data representing the +sched_switch histogram. For each one of those, the referenced +variable's var.hist_data along with the current key is used to look up +the corresponding tracing_map_elt in that histogram. Once found, the +referenced variable's var.idx is used to look up the variable's value +using tracing_map_read_var(elt, var.idx), which yields the value of +the variable for that element, ts0 in the case above. Note that both +the hist_fields representing both the variable and the variable +reference have the same var.idx, so this is straightforward. + +Variable and variable reference test +------------------------------------ + +This example creates a variable on the sched_waking event, ts0, and +uses it in the sched_switch trigger. The sched_switch trigger also +creates its own variable, wakeup_lat, but nothing yet uses it:: + + # echo 'hist:keys=pid:ts0=common_timestamp.usecs' >> events/sched/sched_waking/trigger + + # echo 'hist:keys=next_pid:wakeup_lat=common_timestamp.usecs-$ts0' >> events/sched/sched_switch/trigger + +Looking at the sched_waking 'hist_debug' output, in addition to the +normal key and value hist_fields, in the val fields section we see a +field with the HIST_FIELD_FL_VAR flag, which indicates that that field +represents a variable. Note that in addition to the variable name, +contained in the var.name field, it includes the var.idx, which is the +index into the tracing_map_elt.vars[] array of the actual variable +location. Note also that the output shows that variables live in the +same part of the hist_data->fields[] array as normal values:: + + # cat events/sched/sched_waking/hist_debug + + # event histogram + # + # trigger info: hist:keys=pid:vals=hitcount:ts0=common_timestamp.usecs:sort=hitcount:size=2048:clock=global [active] + # + + hist_data: 000000009536f554 + + n_vals: 2 + n_keys: 1 + n_fields: 3 + + val fields: + + hist_data->fields[0]: + flags: + VAL: HIST_FIELD_FL_HITCOUNT + type: u64 + size: 8 + is_signed: 0 + + hist_data->fields[1]: + flags: + HIST_FIELD_FL_VAR + var.name: ts0 + var.idx (into tracing_map_elt.vars[]): 0 + type: u64 + size: 8 + is_signed: 0 + + key fields: + + hist_data->fields[2]: + flags: + HIST_FIELD_FL_KEY + ftrace_event_field name: pid + type: pid_t + size: 8 + is_signed: 1 + +Moving on to the sched_switch trigger hist_debug output, in addition +to the unused wakeup_lat variable, we see a new section displaying +variable references. Variable references are displayed in a separate +section because in addition to being logically separate from +variables and values, they actually live in a separate hist_data +array, var_refs[]. + +In this example, the sched_switch trigger has a reference to a +variable on the sched_waking trigger, $ts0. Looking at the details, +we can see that the var.hist_data value of the referenced variable +matches the previously displayed sched_waking trigger, and the var.idx +value matches the previously displayed var.idx value for that +variable. Also displayed is the var_ref_idx value for that variable +reference, which is where the value for that variable is cached for +use when the trigger is invoked:: + + # cat events/sched/sched_switch/hist_debug + + # event histogram + # + # trigger info: hist:keys=next_pid:vals=hitcount:wakeup_lat=common_timestamp.usecs-$ts0:sort=hitcount:size=2048:clock=global [active] + # + + hist_data: 00000000f4ee8006 + + n_vals: 2 + n_keys: 1 + n_fields: 3 + + val fields: + + hist_data->fields[0]: + flags: + VAL: HIST_FIELD_FL_HITCOUNT + type: u64 + size: 8 + is_signed: 0 + + hist_data->fields[1]: + flags: + HIST_FIELD_FL_VAR + var.name: wakeup_lat + var.idx (into tracing_map_elt.vars[]): 0 + type: u64 + size: 0 + is_signed: 0 + + key fields: + + hist_data->fields[2]: + flags: + HIST_FIELD_FL_KEY + ftrace_event_field name: next_pid + type: pid_t + size: 8 + is_signed: 1 + + variable reference fields: + + hist_data->var_refs[0]: + flags: + HIST_FIELD_FL_VAR_REF + name: ts0 + var.idx (into tracing_map_elt.vars[]): 0 + var.hist_data: 000000009536f554 + var_ref_idx (into hist_data->var_refs[]): 0 + type: u64 + size: 8 + is_signed: 0 + +The commands below can be used to clean things up for the next test:: + + # echo '!hist:keys=next_pid:wakeup_lat=common_timestamp.usecs-$ts0' >> events/sched/sched_switch/trigger + + # echo '!hist:keys=pid:ts0=common_timestamp.usecs' >> events/sched/sched_waking/trigger + +Actions and Handlers +==================== + +Adding onto the previous example, we will now do something with that +wakeup_lat variable, namely send it and another field as a synthetic +event. + +The onmatch() action below basically says that whenever we have a +sched_switch event, if we have a matching sched_waking event, in this +case if we have a pid in the sched_waking histogram that matches the +next_pid field on this sched_switch event, we retrieve the +variables specified in the wakeup_latency() trace action, and use +them to generate a new wakeup_latency event into the trace stream. + +Note that the way the trace handlers such as wakeup_latency() (which +could equivalently be written trace(wakeup_latency,$wakeup_lat,next_pid) +are implemented, the parameters specified to the trace handler must be +variables. In this case, $wakeup_lat is obviously a variable, but +next_pid isn't, since it's just naming a field in the sched_switch +trace event. Since this is something that almost every trace() and +save() action does, a special shortcut is implemented to allow field +names to be used directly in those cases. How it works is that under +the covers, a temporary variable is created for the named field, and +this variable is what is actually passed to the trace handler. In the +code and documentation, this type of variable is called a 'field +variable'. + +Fields on other trace event's histograms can be used as well. In that +case we have to generate a new histogram and an unfortunately named +'synthetic_field' (the use of synthetic here has nothing to do with +synthetic events) and use that special histogram field as a variable. + +The diagram below illustrates the new elements described above in the +context of the sched_switch histogram using the onmatch() handler and +the trace() action. + +First, we define the wakeup_latency synthetic event:: + + # echo 'wakeup_latency u64 lat; pid_t pid' >> synthetic_events + +Next, the sched_waking hist trigger as before:: + + # echo 'hist:keys=pid:ts0=common_timestamp.usecs' >> + events/sched/sched_waking/trigger + +Finally, we create a hist trigger on the sched_switch event that +generates a wakeup_latency() trace event. In this case we pass +next_pid into the wakeup_latency synthetic event invocation, which +means it will be automatically converted into a field variable:: + + # echo 'hist:keys=next_pid:wakeup_lat=common_timestamp.usecs-$ts0: \ + onmatch(sched.sched_waking).wakeup_latency($wakeup_lat,next_pid)' >> + /sys/kernel/debug/tracing/events/sched/sched_switch/trigger + +The diagram for the sched_switch event is similar to previous examples +but shows the additional field_vars[] array for hist_data and shows +the linkages between the field_vars and the variables and references +created to implement the field variables. The details are discussed +below:: + + +------------------+ + | hist_data | + +------------------+ +-----------------------+ + | .fields[] |-->| val = hitcount | + +----------------+ +-----------------------+ + | .map | | .size | + +----------------+ +---------------------+ + +---| .field_vars[] | | .offset | + | +----------------+ +---------------------+ + |+--| .var_refs[] | | .offset | + || +----------------+ +---------------------+ + || | .fn() | + || var_ref_vals[] +---------------------+ + || +-------------+ | .flags | + || | $ts0 |<---+ +---------------------+ + || +-------------+ | | .var.idx | + || | $next_pid |<-+ | +---------------------+ + || +-------------+ | | | .var.hist_data | + ||+>| $wakeup_lat | | | +---------------------+ + ||| +-------------+ | | | .var_ref_idx | + ||| | | | | +-----------------------+ + ||| +-------------+ | | | var = wakeup_lat | + ||| . | | +-----------------------+ + ||| . | | | .size | + ||| . | | +---------------------+ + ||| +-------------+ | | | .offset | + ||| | | | | +---------------------+ + ||| +-------------+ | | | .fn() | + ||| | | | | +---------------------+ + ||| +-------------+ | | | .flags & FL_VAR | + ||| | | +---------------------+ + ||| | | | .var.idx | + ||| | | +---------------------+ + ||| | | | .var.hist_data | + ||| | | +---------------------+ + ||| | | | .var_ref_idx | + ||| | | +---------------------+ + ||| | | . + ||| | | . + ||| | | . + ||| | | . + ||| +--------------+ | | . + +-->| field_var | | | . + || +--------------+ | | . + || | var | | | . + || +------------+ | | . + || | val | | | . + || +--------------+ | | . + || | field_var | | | . + || +--------------+ | | . + || | var | | | . + || +------------+ | | . + || | val | | | . + || +------------+ | | . + || . | | . + || . | | . + || . | | +-----------------------+ <--- n_vals + || +--------------+ | | | key = pid | + || | field_var | | | +-----------------------+ + || +--------------+ | | | .size | + || | var |--+| +---------------------+ + || +------------+ ||| | .offset | + || | val |-+|| +---------------------+ + || +------------+ ||| | .fn() | + || ||| +---------------------+ + || ||| | .flags | + || ||| +---------------------+ + || ||| | .var.idx | + || ||| +---------------------+ <--- n_fields + || ||| + || ||| n_keys = n_fields - n_vals + || ||| +-----------------------+ + || |+->| var = next_pid | + || | | +-----------------------+ + || | | | .size | + || | | +---------------------+ + || | | | .offset | + || | | +---------------------+ + || | | | .flags & FL_VAR | + || | | +---------------------+ + || | | | .var.idx | + || | | +---------------------+ + || | | | .var.hist_data | + || | | +-----------------------+ + || +-->| val for next_pid | + || | | +-----------------------+ + || | | | .size | + || | | +---------------------+ + || | | | .offset | + || | | +---------------------+ + || | | | .fn() | + || | | +---------------------+ + || | | | .flags | + || | | +---------------------+ + || | | | | + || | | +---------------------+ + || | | + || | | + || | | +-----------------------+ + +|------------------|-|>| var_ref = $ts0 | + | | | +-----------------------+ + | | | | .size | + | | | +---------------------+ + | | | | .offset | + | | | +---------------------+ + | | | | .fn() | + | | | +---------------------+ + | | | | .flags & FL_VAR_REF | + | | | +---------------------+ + | | +---| .var_ref_idx | + | | +-----------------------+ + | | | var_ref = $next_pid | + | | +-----------------------+ + | | | .size | + | | +---------------------+ + | | | .offset | + | | +---------------------+ + | | | .fn() | + | | +---------------------+ + | | | .flags & FL_VAR_REF | + | | +---------------------+ + | +-----| .var_ref_idx | + | +-----------------------+ + | | var_ref = $wakeup_lat | + | +-----------------------+ + | | .size | + | +---------------------+ + | | .offset | + | +---------------------+ + | | .fn() | + | +---------------------+ + | | .flags & FL_VAR_REF | + | +---------------------+ + +------------------------| .var_ref_idx | + +---------------------+ + +As you can see, for a field variable, two hist_fields are created: one +representing the variable, in this case next_pid, and one to actually +get the value of the field from the trace stream, like a normal val +field does. These are created separately from normal variable +creation and are saved in the hist_data->field_vars[] array. See +below for how these are used. In addition, a reference hist_field is +also created, which is needed to reference the field variables such as +$next_pid variable in the trace() action. + +Note that $wakeup_lat is also a variable reference, referencing the +value of the expression common_timestamp-$ts0, and so also needs to +have a hist field entry representing that reference created. + +When hist_trigger_elt_update() is called to get the normal key and +value fields, it also calls update_field_vars(), which goes through +each field_var created for the histogram, and available from +hist_data->field_vars and calls val->fn() to get the data from the +current trace record, and then uses the var's var.idx to set the +variable at the var.idx offset in the appropriate tracing_map_elt's +variable at elt->vars[var.idx]. + +Once all the variables have been updated, resolve_var_refs() can be +called from event_hist_trigger(), and not only can our $ts0 and +$next_pid references be resolved but the $wakeup_lat reference as +well. At this point, the trace() action can simply access the values +assembled in the var_ref_vals[] array and generate the trace event. + +The same process occurs for the field variables associated with the +save() action. + +Abbreviations used in the diagram:: + + hist_data = struct hist_trigger_data + hist_data.fields = struct hist_field + field_var = struct field_var + fn = hist_field_fn_t + FL_KEY = HIST_FIELD_FL_KEY + FL_VAR = HIST_FIELD_FL_VAR + FL_VAR_REF = HIST_FIELD_FL_VAR_REF + +trace() action field variable test +---------------------------------- + +This example adds to the previous test example by finally making use +of the wakeup_lat variable, but in addition also creates a couple of +field variables that then are all passed to the wakeup_latency() trace +action via the onmatch() handler. + +First, we create the wakeup_latency synthetic event:: + + # echo 'wakeup_latency u64 lat; pid_t pid; char comm[16]' >> synthetic_events + +Next, the sched_waking trigger from previous examples:: + + # echo 'hist:keys=pid:ts0=common_timestamp.usecs' >> events/sched/sched_waking/trigger + +Finally, as in the previous test example, we calculate and assign the +wakeup latency using the $ts0 reference from the sched_waking trigger +to the wakeup_lat variable, and finally use it along with a couple +sched_switch event fields, next_pid and next_comm, to generate a +wakeup_latency trace event. The next_pid and next_comm event fields +are automatically converted into field variables for this purpose:: + + # echo 'hist:keys=next_pid:wakeup_lat=common_timestamp.usecs-$ts0:onmatch(sched.sched_waking).wakeup_latency($wakeup_lat,next_pid,next_comm)' >> /sys/kernel/debug/tracing/events/sched/sched_switch/trigger + +The sched_waking hist_debug output shows the same data as in the +previous test example:: + + # cat events/sched/sched_waking/hist_debug + + # event histogram + # + # trigger info: hist:keys=pid:vals=hitcount:ts0=common_timestamp.usecs:sort=hitcount:size=2048:clock=global [active] + # + + hist_data: 00000000d60ff61f + + n_vals: 2 + n_keys: 1 + n_fields: 3 + + val fields: + + hist_data->fields[0]: + flags: + VAL: HIST_FIELD_FL_HITCOUNT + type: u64 + size: 8 + is_signed: 0 + + hist_data->fields[1]: + flags: + HIST_FIELD_FL_VAR + var.name: ts0 + var.idx (into tracing_map_elt.vars[]): 0 + type: u64 + size: 8 + is_signed: 0 + + key fields: + + hist_data->fields[2]: + flags: + HIST_FIELD_FL_KEY + ftrace_event_field name: pid + type: pid_t + size: 8 + is_signed: 1 + +The sched_switch hist_debug output shows the same key and value fields +as in the previous test example - note that wakeup_lat is still in the +val fields section, but that the new field variables are not there - +although the field variables are variables, they're held separately in +the hist_data's field_vars[] array. Although the field variables and +the normal variables are located in separate places, you can see that +the actual variable locations for those variables in the +tracing_map_elt.vars[] do have increasing indices as expected: +wakeup_lat takes the var.idx = 0 slot, while the field variables for +next_pid and next_comm have values var.idx = 1, and var.idx = 2. Note +also that those are the same values displayed for the variable +references corresponding to those variables in the variable reference +fields section. Since there are two triggers and thus two hist_data +addresses, those addresses also need to be accounted for when doing +the matching - you can see that the first variable refers to the 0 +var.idx on the previous hist trigger (see the hist_data address +associated with that trigger), while the second variable refers to the +0 var.idx on the sched_switch hist trigger, as do all the remaining +variable references. + +Finally, the action tracking variables section just shows the system +and event name for the onmatch() handler:: + + # cat events/sched/sched_switch/hist_debug + + # event histogram + # + # trigger info: hist:keys=next_pid:vals=hitcount:wakeup_lat=common_timestamp.usecs-$ts0:sort=hitcount:size=2048:clock=global:onmatch(sched.sched_waking).wakeup_latency($wakeup_lat,next_pid,next_comm) [active] + # + + hist_data: 0000000008f551b7 + + n_vals: 2 + n_keys: 1 + n_fields: 3 + + val fields: + + hist_data->fields[0]: + flags: + VAL: HIST_FIELD_FL_HITCOUNT + type: u64 + size: 8 + is_signed: 0 + + hist_data->fields[1]: + flags: + HIST_FIELD_FL_VAR + var.name: wakeup_lat + var.idx (into tracing_map_elt.vars[]): 0 + type: u64 + size: 0 + is_signed: 0 + + key fields: + + hist_data->fields[2]: + flags: + HIST_FIELD_FL_KEY + ftrace_event_field name: next_pid + type: pid_t + size: 8 + is_signed: 1 + + variable reference fields: + + hist_data->var_refs[0]: + flags: + HIST_FIELD_FL_VAR_REF + name: ts0 + var.idx (into tracing_map_elt.vars[]): 0 + var.hist_data: 00000000d60ff61f + var_ref_idx (into hist_data->var_refs[]): 0 + type: u64 + size: 8 + is_signed: 0 + + hist_data->var_refs[1]: + flags: + HIST_FIELD_FL_VAR_REF + name: wakeup_lat + var.idx (into tracing_map_elt.vars[]): 0 + var.hist_data: 0000000008f551b7 + var_ref_idx (into hist_data->var_refs[]): 1 + type: u64 + size: 0 + is_signed: 0 + + hist_data->var_refs[2]: + flags: + HIST_FIELD_FL_VAR_REF + name: next_pid + var.idx (into tracing_map_elt.vars[]): 1 + var.hist_data: 0000000008f551b7 + var_ref_idx (into hist_data->var_refs[]): 2 + type: pid_t + size: 4 + is_signed: 0 + + hist_data->var_refs[3]: + flags: + HIST_FIELD_FL_VAR_REF + name: next_comm + var.idx (into tracing_map_elt.vars[]): 2 + var.hist_data: 0000000008f551b7 + var_ref_idx (into hist_data->var_refs[]): 3 + type: char[16] + size: 256 + is_signed: 0 + + field variables: + + hist_data->field_vars[0]: + + field_vars[0].var: + flags: + HIST_FIELD_FL_VAR + var.name: next_pid + var.idx (into tracing_map_elt.vars[]): 1 + + field_vars[0].val: + ftrace_event_field name: next_pid + type: pid_t + size: 4 + is_signed: 1 + + hist_data->field_vars[1]: + + field_vars[1].var: + flags: + HIST_FIELD_FL_VAR + var.name: next_comm + var.idx (into tracing_map_elt.vars[]): 2 + + field_vars[1].val: + ftrace_event_field name: next_comm + type: char[16] + size: 256 + is_signed: 0 + + action tracking variables (for onmax()/onchange()/onmatch()): + + hist_data->actions[0].match_data.event_system: sched + hist_data->actions[0].match_data.event: sched_waking + +The commands below can be used to clean things up for the next test:: + + # echo '!hist:keys=next_pid:wakeup_lat=common_timestamp.usecs-$ts0:onmatch(sched.sched_waking).wakeup_latency($wakeup_lat,next_pid,next_comm)' >> /sys/kernel/debug/tracing/events/sched/sched_switch/trigger + + # echo '!hist:keys=pid:ts0=common_timestamp.usecs' >> events/sched/sched_waking/trigger + + # echo '!wakeup_latency u64 lat; pid_t pid; char comm[16]' >> synthetic_events + +action_data and the trace() action +---------------------------------- + +As mentioned above, when the trace() action generates a synthetic +event, all the parameters to the synthetic event either already are +variables or are converted into variables (via field variables), and +finally all those variable values are collected via references to them +into a var_ref_vals[] array. + +The values in the var_ref_vals[] array, however, don't necessarily +follow the same ordering as the synthetic event params. To address +that, struct action_data contains another array, var_ref_idx[] that +maps the trace action params to the var_ref_vals[] values. Below is a +diagram illustrating that for the wakeup_latency() synthetic event:: + + +------------------+ wakeup_latency() + | action_data | event params var_ref_vals[] + +------------------+ +-----------------+ +-----------------+ + | .var_ref_idx[] |--->| $wakeup_lat idx |---+ | | + +----------------+ +-----------------+ | +-----------------+ + | .synth_event | | $next_pid idx |---|-+ | $wakeup_lat val | + +----------------+ +-----------------+ | | +-----------------+ + . | +->| $next_pid val | + . | +-----------------+ + . | . + +-----------------+ | . + | | | . + +-----------------+ | +-----------------+ + +--->| $wakeup_lat val | + +-----------------+ + +Basically, how this ends up getting used in the synthetic event probe +function, trace_event_raw_event_synth(), is as follows:: + + for each field i in .synth_event + val_idx = .var_ref_idx[i] + val = var_ref_vals[val_idx] + +action_data and the onXXX() handlers +------------------------------------ + +The hist trigger onXXX() actions other than onmatch(), such as onmax() +and onchange(), also make use of and internally create hidden +variables. This information is contained in the +action_data.track_data struct, and is also visible in the hist_debug +output as will be described in the example below. + +Typically, the onmax() or onchange() handlers are used in conjunction +with the save() and snapshot() actions. For example:: + + # echo 'hist:keys=next_pid:wakeup_lat=common_timestamp.usecs-$ts0: \ + onmax($wakeup_lat).save(next_comm,prev_pid,prev_prio,prev_comm)' >> + /sys/kernel/debug/tracing/events/sched/sched_switch/trigger + +or:: + + # echo 'hist:keys=next_pid:wakeup_lat=common_timestamp.usecs-$ts0: \ + onmax($wakeup_lat).snapshot()' >> + /sys/kernel/debug/tracing/events/sched/sched_switch/trigger + +save() action field variable test +--------------------------------- + +For this example, instead of generating a synthetic event, the save() +action is used to save field values whenever an onmax() handler +detects that a new max latency has been hit. As in the previous +example, the values being saved are also field values, but in this +case, are kept in a separate hist_data array named save_vars[]. + +As in previous test examples, we set up the sched_waking trigger:: + + # echo 'hist:keys=pid:ts0=common_timestamp.usecs' >> events/sched/sched_waking/trigger + +In this case, however, we set up the sched_switch trigger to save some +sched_switch field values whenever we hit a new maximum latency. For +both the onmax() handler and save() action, variables will be created, +which we can use the hist_debug files to examine:: + + # echo 'hist:keys=next_pid:wakeup_lat=common_timestamp.usecs-$ts0:onmax($wakeup_lat).save(next_comm,prev_pid,prev_prio,prev_comm)' >> events/sched/sched_switch/trigger + +The sched_waking hist_debug output shows the same data as in the +previous test examples:: + + # cat events/sched/sched_waking/hist_debug + + # + # trigger info: hist:keys=pid:vals=hitcount:ts0=common_timestamp.usecs:sort=hitcount:size=2048:clock=global [active] + # + + hist_data: 00000000e6290f48 + + n_vals: 2 + n_keys: 1 + n_fields: 3 + + val fields: + + hist_data->fields[0]: + flags: + VAL: HIST_FIELD_FL_HITCOUNT + type: u64 + size: 8 + is_signed: 0 + + hist_data->fields[1]: + flags: + HIST_FIELD_FL_VAR + var.name: ts0 + var.idx (into tracing_map_elt.vars[]): 0 + type: u64 + size: 8 + is_signed: 0 + + key fields: + + hist_data->fields[2]: + flags: + HIST_FIELD_FL_KEY + ftrace_event_field name: pid + type: pid_t + size: 8 + is_signed: 1 + +The output of the sched_switch trigger shows the same val and key +values as before, but also shows a couple new sections. + +First, the action tracking variables section now shows the +actions[].track_data information describing the special tracking +variables and references used to track, in this case, the running +maximum value. The actions[].track_data.var_ref member contains the +reference to the variable being tracked, in this case the $wakeup_lat +variable. In order to perform the onmax() handler function, there +also needs to be a variable that tracks the current maximum by getting +updated whenever a new maximum is hit. In this case, we can see that +an auto-generated variable named ' __max' has been created and is +visible in the actions[].track_data.track_var variable. + +Finally, in the new 'save action variables' section, we can see that +the 4 params to the save() function have resulted in 4 field variables +being created for the purposes of saving the values of the named +fields when the max is hit. These variables are kept in a separate +save_vars[] array off of hist_data, so are displayed in a separate +section:: + + # cat events/sched/sched_switch/hist_debug + + # event histogram + # + # trigger info: hist:keys=next_pid:vals=hitcount:wakeup_lat=common_timestamp.usecs-$ts0:sort=hitcount:size=2048:clock=global:onmax($wakeup_lat).save(next_comm,prev_pid,prev_prio,prev_comm) [active] + # + + hist_data: 0000000057bcd28d + + n_vals: 2 + n_keys: 1 + n_fields: 3 + + val fields: + + hist_data->fields[0]: + flags: + VAL: HIST_FIELD_FL_HITCOUNT + type: u64 + size: 8 + is_signed: 0 + + hist_data->fields[1]: + flags: + HIST_FIELD_FL_VAR + var.name: wakeup_lat + var.idx (into tracing_map_elt.vars[]): 0 + type: u64 + size: 0 + is_signed: 0 + + key fields: + + hist_data->fields[2]: + flags: + HIST_FIELD_FL_KEY + ftrace_event_field name: next_pid + type: pid_t + size: 8 + is_signed: 1 + + variable reference fields: + + hist_data->var_refs[0]: + flags: + HIST_FIELD_FL_VAR_REF + name: ts0 + var.idx (into tracing_map_elt.vars[]): 0 + var.hist_data: 00000000e6290f48 + var_ref_idx (into hist_data->var_refs[]): 0 + type: u64 + size: 8 + is_signed: 0 + + hist_data->var_refs[1]: + flags: + HIST_FIELD_FL_VAR_REF + name: wakeup_lat + var.idx (into tracing_map_elt.vars[]): 0 + var.hist_data: 0000000057bcd28d + var_ref_idx (into hist_data->var_refs[]): 1 + type: u64 + size: 0 + is_signed: 0 + + action tracking variables (for onmax()/onchange()/onmatch()): + + hist_data->actions[0].track_data.var_ref: + flags: + HIST_FIELD_FL_VAR_REF + name: wakeup_lat + var.idx (into tracing_map_elt.vars[]): 0 + var.hist_data: 0000000057bcd28d + var_ref_idx (into hist_data->var_refs[]): 1 + type: u64 + size: 0 + is_signed: 0 + + hist_data->actions[0].track_data.track_var: + flags: + HIST_FIELD_FL_VAR + var.name: __max + var.idx (into tracing_map_elt.vars[]): 1 + type: u64 + size: 8 + is_signed: 0 + + save action variables (save() params): + + hist_data->save_vars[0]: + + save_vars[0].var: + flags: + HIST_FIELD_FL_VAR + var.name: next_comm + var.idx (into tracing_map_elt.vars[]): 2 + + save_vars[0].val: + ftrace_event_field name: next_comm + type: char[16] + size: 256 + is_signed: 0 + + hist_data->save_vars[1]: + + save_vars[1].var: + flags: + HIST_FIELD_FL_VAR + var.name: prev_pid + var.idx (into tracing_map_elt.vars[]): 3 + + save_vars[1].val: + ftrace_event_field name: prev_pid + type: pid_t + size: 4 + is_signed: 1 + + hist_data->save_vars[2]: + + save_vars[2].var: + flags: + HIST_FIELD_FL_VAR + var.name: prev_prio + var.idx (into tracing_map_elt.vars[]): 4 + + save_vars[2].val: + ftrace_event_field name: prev_prio + type: int + size: 4 + is_signed: 1 + + hist_data->save_vars[3]: + + save_vars[3].var: + flags: + HIST_FIELD_FL_VAR + var.name: prev_comm + var.idx (into tracing_map_elt.vars[]): 5 + + save_vars[3].val: + ftrace_event_field name: prev_comm + type: char[16] + size: 256 + is_signed: 0 + +The commands below can be used to clean things up for the next test:: + + # echo '!hist:keys=next_pid:wakeup_lat=common_timestamp.usecs-$ts0:onmax($wakeup_lat).save(next_comm,prev_pid,prev_prio,prev_comm)' >> events/sched/sched_switch/trigger + + # echo '!hist:keys=pid:ts0=common_timestamp.usecs' >> events/sched/sched_waking/trigger + +A couple special cases +====================== + +While the above covers the basics of the histogram internals, there +are a couple of special cases that should be discussed, since they +tend to create even more confusion. Those are field variables on other +histograms, and aliases, both described below through example tests +using the hist_debug files. + +Test of field variables on other histograms +------------------------------------------- + +This example is similar to the previous examples, but in this case, +the sched_switch trigger references a hist trigger field on another +event, namely the sched_waking event. In order to accomplish this, a +field variable is created for the other event, but since an existing +histogram can't be used, as existing histograms are immutable, a new +histogram with a matching variable is created and used, and we'll see +that reflected in the hist_debug output shown below. + +First, we create the wakeup_latency synthetic event. Note the +addition of the prio field:: + + # echo 'wakeup_latency u64 lat; pid_t pid; int prio' >> synthetic_events + +As in previous test examples, we set up the sched_waking trigger:: + + # echo 'hist:keys=pid:ts0=common_timestamp.usecs' >> events/sched/sched_waking/trigger + +Here we set up a hist trigger on sched_switch to send a wakeup_latency +event using an onmatch handler naming the sched_waking event. Note +that the third param being passed to the wakeup_latency() is prio, +which is a field name that needs to have a field variable created for +it. There isn't however any prio field on the sched_switch event so +it would seem that it wouldn't be possible to create a field variable +for it. The matching sched_waking event does have a prio field, so it +should be possible to make use of it for this purpose. The problem +with that is that it's not currently possible to define a new variable +on an existing histogram, so it's not possible to add a new prio field +variable to the existing sched_waking histogram. It is however +possible to create an additional new 'matching' sched_waking histogram +for the same event, meaning that it uses the same key and filters, and +define the new prio field variable on that. + +Here's the sched_switch trigger:: + + # echo 'hist:keys=next_pid:wakeup_lat=common_timestamp.usecs-$ts0:onmatch(sched.sched_waking).wakeup_latency($wakeup_lat,next_pid,prio)' >> events/sched/sched_switch/trigger + +And here's the output of the hist_debug information for the +sched_waking hist trigger. Note that there are two histograms +displayed in the output: the first is the normal sched_waking +histogram we've seen in the previous examples, and the second is the +special histogram we created to provide the prio field variable. + +Looking at the second histogram below, we see a variable with the name +synthetic_prio. This is the field variable created for the prio field +on that sched_waking histogram:: + + # cat events/sched/sched_waking/hist_debug + + # event histogram + # + # trigger info: hist:keys=pid:vals=hitcount:ts0=common_timestamp.usecs:sort=hitcount:size=2048:clock=global [active] + # + + hist_data: 00000000349570e4 + + n_vals: 2 + n_keys: 1 + n_fields: 3 + + val fields: + + hist_data->fields[0]: + flags: + VAL: HIST_FIELD_FL_HITCOUNT + type: u64 + size: 8 + is_signed: 0 + + hist_data->fields[1]: + flags: + HIST_FIELD_FL_VAR + var.name: ts0 + var.idx (into tracing_map_elt.vars[]): 0 + type: u64 + size: 8 + is_signed: 0 + + key fields: + + hist_data->fields[2]: + flags: + HIST_FIELD_FL_KEY + ftrace_event_field name: pid + type: pid_t + size: 8 + is_signed: 1 + + + # event histogram + # + # trigger info: hist:keys=pid:vals=hitcount:synthetic_prio=prio:sort=hitcount:size=2048 [active] + # + + hist_data: 000000006920cf38 + + n_vals: 2 + n_keys: 1 + n_fields: 3 + + val fields: + + hist_data->fields[0]: + flags: + VAL: HIST_FIELD_FL_HITCOUNT + type: u64 + size: 8 + is_signed: 0 + + hist_data->fields[1]: + flags: + HIST_FIELD_FL_VAR + ftrace_event_field name: prio + var.name: synthetic_prio + var.idx (into tracing_map_elt.vars[]): 0 + type: int + size: 4 + is_signed: 1 + + key fields: + + hist_data->fields[2]: + flags: + HIST_FIELD_FL_KEY + ftrace_event_field name: pid + type: pid_t + size: 8 + is_signed: 1 + +Looking at the sched_switch histogram below, we can see a reference to +the synthetic_prio variable on sched_waking, and looking at the +associated hist_data address we see that it is indeed associated with +the new histogram. Note also that the other references are to a +normal variable, wakeup_lat, and to a normal field variable, next_pid, +the details of which are in the field variables section:: + + # cat events/sched/sched_switch/hist_debug + + # event histogram + # + # trigger info: hist:keys=next_pid:vals=hitcount:wakeup_lat=common_timestamp.usecs-$ts0:sort=hitcount:size=2048:clock=global:onmatch(sched.sched_waking).wakeup_latency($wakeup_lat,next_pid,prio) [active] + # + + hist_data: 00000000a73b67df + + n_vals: 2 + n_keys: 1 + n_fields: 3 + + val fields: + + hist_data->fields[0]: + flags: + VAL: HIST_FIELD_FL_HITCOUNT + type: u64 + size: 8 + is_signed: 0 + + hist_data->fields[1]: + flags: + HIST_FIELD_FL_VAR + var.name: wakeup_lat + var.idx (into tracing_map_elt.vars[]): 0 + type: u64 + size: 0 + is_signed: 0 + + key fields: + + hist_data->fields[2]: + flags: + HIST_FIELD_FL_KEY + ftrace_event_field name: next_pid + type: pid_t + size: 8 + is_signed: 1 + + variable reference fields: + + hist_data->var_refs[0]: + flags: + HIST_FIELD_FL_VAR_REF + name: ts0 + var.idx (into tracing_map_elt.vars[]): 0 + var.hist_data: 00000000349570e4 + var_ref_idx (into hist_data->var_refs[]): 0 + type: u64 + size: 8 + is_signed: 0 + + hist_data->var_refs[1]: + flags: + HIST_FIELD_FL_VAR_REF + name: wakeup_lat + var.idx (into tracing_map_elt.vars[]): 0 + var.hist_data: 00000000a73b67df + var_ref_idx (into hist_data->var_refs[]): 1 + type: u64 + size: 0 + is_signed: 0 + + hist_data->var_refs[2]: + flags: + HIST_FIELD_FL_VAR_REF + name: next_pid + var.idx (into tracing_map_elt.vars[]): 1 + var.hist_data: 00000000a73b67df + var_ref_idx (into hist_data->var_refs[]): 2 + type: pid_t + size: 4 + is_signed: 0 + + hist_data->var_refs[3]: + flags: + HIST_FIELD_FL_VAR_REF + name: synthetic_prio + var.idx (into tracing_map_elt.vars[]): 0 + var.hist_data: 000000006920cf38 + var_ref_idx (into hist_data->var_refs[]): 3 + type: int + size: 4 + is_signed: 1 + + field variables: + + hist_data->field_vars[0]: + + field_vars[0].var: + flags: + HIST_FIELD_FL_VAR + var.name: next_pid + var.idx (into tracing_map_elt.vars[]): 1 + + field_vars[0].val: + ftrace_event_field name: next_pid + type: pid_t + size: 4 + is_signed: 1 + + action tracking variables (for onmax()/onchange()/onmatch()): + + hist_data->actions[0].match_data.event_system: sched + hist_data->actions[0].match_data.event: sched_waking + +The commands below can be used to clean things up for the next test:: + + # echo '!hist:keys=next_pid:wakeup_lat=common_timestamp.usecs-$ts0:onmatch(sched.sched_waking).wakeup_latency($wakeup_lat,next_pid,prio)' >> events/sched/sched_switch/trigger + + # echo '!hist:keys=pid:ts0=common_timestamp.usecs' >> events/sched/sched_waking/trigger + + # echo '!wakeup_latency u64 lat; pid_t pid; int prio' >> synthetic_events + +Alias test +---------- + +This example is very similar to previous examples, but demonstrates +the alias flag. + +First, we create the wakeup_latency synthetic event:: + + # echo 'wakeup_latency u64 lat; pid_t pid; char comm[16]' >> synthetic_events + +Next, we create a sched_waking trigger similar to previous examples, +but in this case we save the pid in the waking_pid variable:: + + # echo 'hist:keys=pid:waking_pid=pid:ts0=common_timestamp.usecs' >> events/sched/sched_waking/trigger + +For the sched_switch trigger, instead of using $waking_pid directly in +the wakeup_latency synthetic event invocation, we create an alias of +$waking_pid named $woken_pid, and use that in the synthetic event +invocation instead:: + + # echo 'hist:keys=next_pid:woken_pid=$waking_pid:wakeup_lat=common_timestamp.usecs-$ts0:onmatch(sched.sched_waking).wakeup_latency($wakeup_lat,$woken_pid,next_comm)' >> events/sched/sched_switch/trigger + +Looking at the sched_waking hist_debug output, in addition to the +normal fields, we can see the waking_pid variable:: + + # cat events/sched/sched_waking/hist_debug + + # event histogram + # + # trigger info: hist:keys=pid:vals=hitcount:waking_pid=pid,ts0=common_timestamp.usecs:sort=hitcount:size=2048:clock=global [active] + # + + hist_data: 00000000a250528c + + n_vals: 3 + n_keys: 1 + n_fields: 4 + + val fields: + + hist_data->fields[0]: + flags: + VAL: HIST_FIELD_FL_HITCOUNT + type: u64 + size: 8 + is_signed: 0 + + hist_data->fields[1]: + flags: + HIST_FIELD_FL_VAR + ftrace_event_field name: pid + var.name: waking_pid + var.idx (into tracing_map_elt.vars[]): 0 + type: pid_t + size: 4 + is_signed: 1 + + hist_data->fields[2]: + flags: + HIST_FIELD_FL_VAR + var.name: ts0 + var.idx (into tracing_map_elt.vars[]): 1 + type: u64 + size: 8 + is_signed: 0 + + key fields: + + hist_data->fields[3]: + flags: + HIST_FIELD_FL_KEY + ftrace_event_field name: pid + type: pid_t + size: 8 + is_signed: 1 + +The sched_switch hist_debug output shows that a variable named +woken_pid has been created but that it also has the +HIST_FIELD_FL_ALIAS flag set. It also has the HIST_FIELD_FL_VAR flag +set, which is why it appears in the val field section. + +Despite that implementation detail, an alias variable is actually more +like a variable reference; in fact it can be thought of as a reference +to a reference. The implementation copies the var_ref->fn() from the +variable reference being referenced, in this case, the waking_pid +fn(), which is hist_field_var_ref() and makes that the fn() of the +alias. The hist_field_var_ref() fn() requires the var_ref_idx of the +variable reference it's using, so waking_pid's var_ref_idx is also +copied to the alias. The end result is that when the value of alias +is retrieved, in the end it just does the same thing the original +reference would have done and retrieves the same value from the +var_ref_vals[] array. You can verify this in the output by noting +that the var_ref_idx of the alias, in this case woken_pid, is the same +as the var_ref_idx of the reference, waking_pid, in the variable +reference fields section. + +Additionally, once it gets that value, since it is also a variable, it +then saves that value into its var.idx. So the var.idx of the +woken_pid alias is 0, which it fills with the value from var_ref_idx 0 +when its fn() is called to update itself. You'll also notice that +there's a woken_pid var_ref in the variable refs section. That is the +reference to the woken_pid alias variable, and you can see that it +retrieves the value from the same var.idx as the woken_pid alias, 0, +and then in turn saves that value in its own var_ref_idx slot, 3, and +the value at this position is finally what gets assigned to the +$woken_pid slot in the trace event invocation:: + + # cat events/sched/sched_switch/hist_debug + + # event histogram + # + # trigger info: hist:keys=next_pid:vals=hitcount:woken_pid=$waking_pid,wakeup_lat=common_timestamp.usecs-$ts0:sort=hitcount:size=2048:clock=global:onmatch(sched.sched_waking).wakeup_latency($wakeup_lat,$woken_pid,next_comm) [active] + # + + hist_data: 0000000055d65ed0 + + n_vals: 3 + n_keys: 1 + n_fields: 4 + + val fields: + + hist_data->fields[0]: + flags: + VAL: HIST_FIELD_FL_HITCOUNT + type: u64 + size: 8 + is_signed: 0 + + hist_data->fields[1]: + flags: + HIST_FIELD_FL_VAR + HIST_FIELD_FL_ALIAS + var.name: woken_pid + var.idx (into tracing_map_elt.vars[]): 0 + var_ref_idx (into hist_data->var_refs[]): 0 + type: pid_t + size: 4 + is_signed: 1 + + hist_data->fields[2]: + flags: + HIST_FIELD_FL_VAR + var.name: wakeup_lat + var.idx (into tracing_map_elt.vars[]): 1 + type: u64 + size: 0 + is_signed: 0 + + key fields: + + hist_data->fields[3]: + flags: + HIST_FIELD_FL_KEY + ftrace_event_field name: next_pid + type: pid_t + size: 8 + is_signed: 1 + + variable reference fields: + + hist_data->var_refs[0]: + flags: + HIST_FIELD_FL_VAR_REF + name: waking_pid + var.idx (into tracing_map_elt.vars[]): 0 + var.hist_data: 00000000a250528c + var_ref_idx (into hist_data->var_refs[]): 0 + type: pid_t + size: 4 + is_signed: 1 + + hist_data->var_refs[1]: + flags: + HIST_FIELD_FL_VAR_REF + name: ts0 + var.idx (into tracing_map_elt.vars[]): 1 + var.hist_data: 00000000a250528c + var_ref_idx (into hist_data->var_refs[]): 1 + type: u64 + size: 8 + is_signed: 0 + + hist_data->var_refs[2]: + flags: + HIST_FIELD_FL_VAR_REF + name: wakeup_lat + var.idx (into tracing_map_elt.vars[]): 1 + var.hist_data: 0000000055d65ed0 + var_ref_idx (into hist_data->var_refs[]): 2 + type: u64 + size: 0 + is_signed: 0 + + hist_data->var_refs[3]: + flags: + HIST_FIELD_FL_VAR_REF + name: woken_pid + var.idx (into tracing_map_elt.vars[]): 0 + var.hist_data: 0000000055d65ed0 + var_ref_idx (into hist_data->var_refs[]): 3 + type: pid_t + size: 4 + is_signed: 1 + + hist_data->var_refs[4]: + flags: + HIST_FIELD_FL_VAR_REF + name: next_comm + var.idx (into tracing_map_elt.vars[]): 2 + var.hist_data: 0000000055d65ed0 + var_ref_idx (into hist_data->var_refs[]): 4 + type: char[16] + size: 256 + is_signed: 0 + + field variables: + + hist_data->field_vars[0]: + + field_vars[0].var: + flags: + HIST_FIELD_FL_VAR + var.name: next_comm + var.idx (into tracing_map_elt.vars[]): 2 + + field_vars[0].val: + ftrace_event_field name: next_comm + type: char[16] + size: 256 + is_signed: 0 + + action tracking variables (for onmax()/onchange()/onmatch()): + + hist_data->actions[0].match_data.event_system: sched + hist_data->actions[0].match_data.event: sched_waking + +The commands below can be used to clean things up for the next test:: + + # echo '!hist:keys=next_pid:woken_pid=$waking_pid:wakeup_lat=common_timestamp.usecs-$ts0:onmatch(sched.sched_waking).wakeup_latency($wakeup_lat,$woken_pid,next_comm)' >> events/sched/sched_switch/trigger + + # echo '!hist:keys=pid:ts0=common_timestamp.usecs' >> events/sched/sched_waking/trigger + + # echo '!wakeup_latency u64 lat; pid_t pid; char comm[16]' >> synthetic_events |