diff options
Diffstat (limited to 'Documentation/accounting')
| -rw-r--r-- | Documentation/accounting/cgroupstats.rst | 31 | ||||
| -rw-r--r-- | Documentation/accounting/delay-accounting.rst | 130 | ||||
| -rw-r--r-- | Documentation/accounting/index.rst | 14 | ||||
| -rw-r--r-- | Documentation/accounting/psi.rst | 188 | ||||
| -rw-r--r-- | Documentation/accounting/taskstats-struct.rst | 199 | ||||
| -rw-r--r-- | Documentation/accounting/taskstats.rst | 180 |
6 files changed, 742 insertions, 0 deletions
diff --git a/Documentation/accounting/cgroupstats.rst b/Documentation/accounting/cgroupstats.rst new file mode 100644 index 000000000..85186e7d4 --- /dev/null +++ b/Documentation/accounting/cgroupstats.rst @@ -0,0 +1,31 @@ +================== +Control Groupstats +================== + +Control Groupstats is inspired by the discussion at +https://lore.kernel.org/r/461CF883.2030308@sw.ru and implements per cgroup statistics as +suggested by Andrew Morton in https://lore.kernel.org/r/20070411114927.1277d7c9.akpm@linux-foundation.org. + +Per cgroup statistics infrastructure re-uses code from the taskstats +interface. A new set of cgroup operations are registered with commands +and attributes specific to cgroups. It should be very easy to +extend per cgroup statistics, by adding members to the cgroupstats +structure. + +The current model for cgroupstats is a pull, a push model (to post +statistics on interesting events), should be very easy to add. Currently +user space requests for statistics by passing the cgroup path. +Statistics about the state of all the tasks in the cgroup is returned to +user space. + +NOTE: We currently rely on delay accounting for extracting information +about tasks blocked on I/O. If CONFIG_TASK_DELAY_ACCT is disabled, this +information will not be available. + +To extract cgroup statistics a utility very similar to getdelays.c +has been developed, the sample output of the utility is shown below:: + + ~/balbir/cgroupstats # ./getdelays -C "/sys/fs/cgroup/a" + sleeping 1, blocked 0, running 1, stopped 0, uninterruptible 0 + ~/balbir/cgroupstats # ./getdelays -C "/sys/fs/cgroup" + sleeping 155, blocked 0, running 1, stopped 0, uninterruptible 2 diff --git a/Documentation/accounting/delay-accounting.rst b/Documentation/accounting/delay-accounting.rst new file mode 100644 index 000000000..7103b62ba --- /dev/null +++ b/Documentation/accounting/delay-accounting.rst @@ -0,0 +1,130 @@ +================ +Delay accounting +================ + +Tasks encounter delays in execution when they wait +for some kernel resource to become available e.g. a +runnable task may wait for a free CPU to run on. + +The per-task delay accounting functionality measures +the delays experienced by a task while + +a) waiting for a CPU (while being runnable) +b) completion of synchronous block I/O initiated by the task +c) swapping in pages +d) memory reclaim +e) thrashing +f) direct compact +g) write-protect copy + +and makes these statistics available to userspace through +the taskstats interface. + +Such delays provide feedback for setting a task's cpu priority, +io priority and rss limit values appropriately. Long delays for +important tasks could be a trigger for raising its corresponding priority. + +The functionality, through its use of the taskstats interface, also provides +delay statistics aggregated for all tasks (or threads) belonging to a +thread group (corresponding to a traditional Unix process). This is a commonly +needed aggregation that is more efficiently done by the kernel. + +Userspace utilities, particularly resource management applications, can also +aggregate delay statistics into arbitrary groups. To enable this, delay +statistics of a task are available both during its lifetime as well as on its +exit, ensuring continuous and complete monitoring can be done. + + +Interface +--------- + +Delay accounting uses the taskstats interface which is described +in detail in a separate document in this directory. Taskstats returns a +generic data structure to userspace corresponding to per-pid and per-tgid +statistics. The delay accounting functionality populates specific fields of +this structure. See + + include/uapi/linux/taskstats.h + +for a description of the fields pertaining to delay accounting. +It will generally be in the form of counters returning the cumulative +delay seen for cpu, sync block I/O, swapin, memory reclaim, thrash page +cache, direct compact, write-protect copy etc. + +Taking the difference of two successive readings of a given +counter (say cpu_delay_total) for a task will give the delay +experienced by the task waiting for the corresponding resource +in that interval. + +When a task exits, records containing the per-task statistics +are sent to userspace without requiring a command. If it is the last exiting +task of a thread group, the per-tgid statistics are also sent. More details +are given in the taskstats interface description. + +The getdelays.c userspace utility in tools/accounting directory allows simple +commands to be run and the corresponding delay statistics to be displayed. It +also serves as an example of using the taskstats interface. + +Usage +----- + +Compile the kernel with:: + + CONFIG_TASK_DELAY_ACCT=y + CONFIG_TASKSTATS=y + +Delay accounting is disabled by default at boot up. +To enable, add:: + + delayacct + +to the kernel boot options. The rest of the instructions below assume this has +been done. Alternatively, use sysctl kernel.task_delayacct to switch the state +at runtime. Note however that only tasks started after enabling it will have +delayacct information. + +After the system has booted up, use a utility +similar to getdelays.c to access the delays +seen by a given task or a task group (tgid). +The utility also allows a given command to be +executed and the corresponding delays to be +seen. + +General format of the getdelays command:: + + getdelays [-dilv] [-t tgid] [-p pid] + +Get delays, since system boot, for pid 10:: + + # ./getdelays -d -p 10 + (output similar to next case) + +Get sum of delays, since system boot, for all pids with tgid 5:: + + # ./getdelays -d -t 5 + print delayacct stats ON + TGID 5 + + + CPU count real total virtual total delay total delay average + 8 7000000 6872122 3382277 0.423ms + IO count delay total delay average + 0 0 0ms + SWAP count delay total delay average + 0 0 0ms + RECLAIM count delay total delay average + 0 0 0ms + THRASHING count delay total delay average + 0 0 0ms + COMPACT count delay total delay average + 0 0 0ms + WPCOPY count delay total delay average + 0 0 0ms + +Get IO accounting for pid 1, it works only with -p:: + + # ./getdelays -i -p 1 + printing IO accounting + linuxrc: read=65536, write=0, cancelled_write=0 + +The above command can be used with -v to get more debug information. diff --git a/Documentation/accounting/index.rst b/Documentation/accounting/index.rst new file mode 100644 index 000000000..9369d8bf3 --- /dev/null +++ b/Documentation/accounting/index.rst @@ -0,0 +1,14 @@ +.. SPDX-License-Identifier: GPL-2.0 + +========== +Accounting +========== + +.. toctree:: + :maxdepth: 1 + + cgroupstats + delay-accounting + psi + taskstats + taskstats-struct diff --git a/Documentation/accounting/psi.rst b/Documentation/accounting/psi.rst new file mode 100644 index 000000000..df6062eb3 --- /dev/null +++ b/Documentation/accounting/psi.rst @@ -0,0 +1,188 @@ +.. _psi: + +================================ +PSI - Pressure Stall Information +================================ + +:Date: April, 2018 +:Author: Johannes Weiner <hannes@cmpxchg.org> + +When CPU, memory or IO devices are contended, workloads experience +latency spikes, throughput losses, and run the risk of OOM kills. + +Without an accurate measure of such contention, users are forced to +either play it safe and under-utilize their hardware resources, or +roll the dice and frequently suffer the disruptions resulting from +excessive overcommit. + +The psi feature identifies and quantifies the disruptions caused by +such resource crunches and the time impact it has on complex workloads +or even entire systems. + +Having an accurate measure of productivity losses caused by resource +scarcity aids users in sizing workloads to hardware--or provisioning +hardware according to workload demand. + +As psi aggregates this information in realtime, systems can be managed +dynamically using techniques such as load shedding, migrating jobs to +other systems or data centers, or strategically pausing or killing low +priority or restartable batch jobs. + +This allows maximizing hardware utilization without sacrificing +workload health or risking major disruptions such as OOM kills. + +Pressure interface +================== + +Pressure information for each resource is exported through the +respective file in /proc/pressure/ -- cpu, memory, and io. + +The format is as such:: + + some avg10=0.00 avg60=0.00 avg300=0.00 total=0 + full avg10=0.00 avg60=0.00 avg300=0.00 total=0 + +The "some" line indicates the share of time in which at least some +tasks are stalled on a given resource. + +The "full" line indicates the share of time in which all non-idle +tasks are stalled on a given resource simultaneously. In this state +actual CPU cycles are going to waste, and a workload that spends +extended time in this state is considered to be thrashing. This has +severe impact on performance, and it's useful to distinguish this +situation from a state where some tasks are stalled but the CPU is +still doing productive work. As such, time spent in this subset of the +stall state is tracked separately and exported in the "full" averages. + +CPU full is undefined at the system level, but has been reported +since 5.13, so it is set to zero for backward compatibility. + +The ratios (in %) are tracked as recent trends over ten, sixty, and +three hundred second windows, which gives insight into short term events +as well as medium and long term trends. The total absolute stall time +(in us) is tracked and exported as well, to allow detection of latency +spikes which wouldn't necessarily make a dent in the time averages, +or to average trends over custom time frames. + +Monitoring for pressure thresholds +================================== + +Users can register triggers and use poll() to be woken up when resource +pressure exceeds certain thresholds. + +A trigger describes the maximum cumulative stall time over a specific +time window, e.g. 100ms of total stall time within any 500ms window to +generate a wakeup event. + +To register a trigger user has to open psi interface file under +/proc/pressure/ representing the resource to be monitored and write the +desired threshold and time window. The open file descriptor should be +used to wait for trigger events using select(), poll() or epoll(). +The following format is used:: + + <some|full> <stall amount in us> <time window in us> + +For example writing "some 150000 1000000" into /proc/pressure/memory +would add 150ms threshold for partial memory stall measured within +1sec time window. Writing "full 50000 1000000" into /proc/pressure/io +would add 50ms threshold for full io stall measured within 1sec time window. + +Triggers can be set on more than one psi metric and more than one trigger +for the same psi metric can be specified. However for each trigger a separate +file descriptor is required to be able to poll it separately from others, +therefore for each trigger a separate open() syscall should be made even +when opening the same psi interface file. Write operations to a file descriptor +with an already existing psi trigger will fail with EBUSY. + +Monitors activate only when system enters stall state for the monitored +psi metric and deactivates upon exit from the stall state. While system is +in the stall state psi signal growth is monitored at a rate of 10 times per +tracking window. + +The kernel accepts window sizes ranging from 500ms to 10s, therefore min +monitoring update interval is 50ms and max is 1s. Min limit is set to +prevent overly frequent polling. Max limit is chosen as a high enough number +after which monitors are most likely not needed and psi averages can be used +instead. + +Unprivileged users can also create monitors, with the only limitation that the +window size must be a multiple of 2s, in order to prevent excessive resource +usage. + +When activated, psi monitor stays active for at least the duration of one +tracking window to avoid repeated activations/deactivations when system is +bouncing in and out of the stall state. + +Notifications to the userspace are rate-limited to one per tracking window. + +The trigger will de-register when the file descriptor used to define the +trigger is closed. + +Userspace monitor usage example +=============================== + +:: + + #include <errno.h> + #include <fcntl.h> + #include <stdio.h> + #include <poll.h> + #include <string.h> + #include <unistd.h> + + /* + * Monitor memory partial stall with 1s tracking window size + * and 150ms threshold. + */ + int main() { + const char trig[] = "some 150000 1000000"; + struct pollfd fds; + int n; + + fds.fd = open("/proc/pressure/memory", O_RDWR | O_NONBLOCK); + if (fds.fd < 0) { + printf("/proc/pressure/memory open error: %s\n", + strerror(errno)); + return 1; + } + fds.events = POLLPRI; + + if (write(fds.fd, trig, strlen(trig) + 1) < 0) { + printf("/proc/pressure/memory write error: %s\n", + strerror(errno)); + return 1; + } + + printf("waiting for events...\n"); + while (1) { + n = poll(&fds, 1, -1); + if (n < 0) { + printf("poll error: %s\n", strerror(errno)); + return 1; + } + if (fds.revents & POLLERR) { + printf("got POLLERR, event source is gone\n"); + return 0; + } + if (fds.revents & POLLPRI) { + printf("event triggered!\n"); + } else { + printf("unknown event received: 0x%x\n", fds.revents); + return 1; + } + } + + return 0; + } + +Cgroup2 interface +================= + +In a system with a CONFIG_CGROUP=y kernel and the cgroup2 filesystem +mounted, pressure stall information is also tracked for tasks grouped +into cgroups. Each subdirectory in the cgroupfs mountpoint contains +cpu.pressure, memory.pressure, and io.pressure files; the format is +the same as the /proc/pressure/ files. + +Per-cgroup psi monitors can be specified and used the same way as +system-wide ones. diff --git a/Documentation/accounting/taskstats-struct.rst b/Documentation/accounting/taskstats-struct.rst new file mode 100644 index 000000000..ca90fd489 --- /dev/null +++ b/Documentation/accounting/taskstats-struct.rst @@ -0,0 +1,199 @@ +==================== +The struct taskstats +==================== + +This document contains an explanation of the struct taskstats fields. + +There are three different groups of fields in the struct taskstats: + +1) Common and basic accounting fields + If CONFIG_TASKSTATS is set, the taskstats interface is enabled and + the common fields and basic accounting fields are collected for + delivery at do_exit() of a task. +2) Delay accounting fields + These fields are placed between:: + + /* Delay accounting fields start */ + + and:: + + /* Delay accounting fields end */ + + Their values are collected if CONFIG_TASK_DELAY_ACCT is set. +3) Extended accounting fields + These fields are placed between:: + + /* Extended accounting fields start */ + + and:: + + /* Extended accounting fields end */ + + Their values are collected if CONFIG_TASK_XACCT is set. + +4) Per-task and per-thread context switch count statistics + +5) Time accounting for SMT machines + +6) Extended delay accounting fields for memory reclaim + +Future extension should add fields to the end of the taskstats struct, and +should not change the relative position of each field within the struct. + +:: + + struct taskstats { + +1) Common and basic accounting fields:: + + /* The version number of this struct. This field is always set to + * TAKSTATS_VERSION, which is defined in <linux/taskstats.h>. + * Each time the struct is changed, the value should be incremented. + */ + __u16 version; + + /* The exit code of a task. */ + __u32 ac_exitcode; /* Exit status */ + + /* The accounting flags of a task as defined in <linux/acct.h> + * Defined values are AFORK, ASU, ACOMPAT, ACORE, and AXSIG. + */ + __u8 ac_flag; /* Record flags */ + + /* The value of task_nice() of a task. */ + __u8 ac_nice; /* task_nice */ + + /* The name of the command that started this task. */ + char ac_comm[TS_COMM_LEN]; /* Command name */ + + /* The scheduling discipline as set in task->policy field. */ + __u8 ac_sched; /* Scheduling discipline */ + + __u8 ac_pad[3]; + __u32 ac_uid; /* User ID */ + __u32 ac_gid; /* Group ID */ + __u32 ac_pid; /* Process ID */ + __u32 ac_ppid; /* Parent process ID */ + + /* The time when a task begins, in [secs] since 1970. */ + __u32 ac_btime; /* Begin time [sec since 1970] */ + + /* The elapsed time of a task, in [usec]. */ + __u64 ac_etime; /* Elapsed time [usec] */ + + /* The user CPU time of a task, in [usec]. */ + __u64 ac_utime; /* User CPU time [usec] */ + + /* The system CPU time of a task, in [usec]. */ + __u64 ac_stime; /* System CPU time [usec] */ + + /* The minor page fault count of a task, as set in task->min_flt. */ + __u64 ac_minflt; /* Minor Page Fault Count */ + + /* The major page fault count of a task, as set in task->maj_flt. */ + __u64 ac_majflt; /* Major Page Fault Count */ + + +2) Delay accounting fields:: + + /* Delay accounting fields start + * + * All values, until the comment "Delay accounting fields end" are + * available only if delay accounting is enabled, even though the last + * few fields are not delays + * + * xxx_count is the number of delay values recorded + * xxx_delay_total is the corresponding cumulative delay in nanoseconds + * + * xxx_delay_total wraps around to zero on overflow + * xxx_count incremented regardless of overflow + */ + + /* Delay waiting for cpu, while runnable + * count, delay_total NOT updated atomically + */ + __u64 cpu_count; + __u64 cpu_delay_total; + + /* Following four fields atomically updated using task->delays->lock */ + + /* Delay waiting for synchronous block I/O to complete + * does not account for delays in I/O submission + */ + __u64 blkio_count; + __u64 blkio_delay_total; + + /* Delay waiting for page fault I/O (swap in only) */ + __u64 swapin_count; + __u64 swapin_delay_total; + + /* cpu "wall-clock" running time + * On some architectures, value will adjust for cpu time stolen + * from the kernel in involuntary waits due to virtualization. + * Value is cumulative, in nanoseconds, without a corresponding count + * and wraps around to zero silently on overflow + */ + __u64 cpu_run_real_total; + + /* cpu "virtual" running time + * Uses time intervals seen by the kernel i.e. no adjustment + * for kernel's involuntary waits due to virtualization. + * Value is cumulative, in nanoseconds, without a corresponding count + * and wraps around to zero silently on overflow + */ + __u64 cpu_run_virtual_total; + /* Delay accounting fields end */ + /* version 1 ends here */ + + +3) Extended accounting fields:: + + /* Extended accounting fields start */ + + /* Accumulated RSS usage in duration of a task, in MBytes-usecs. + * The current rss usage is added to this counter every time + * a tick is charged to a task's system time. So, at the end we + * will have memory usage multiplied by system time. Thus an + * average usage per system time unit can be calculated. + */ + __u64 coremem; /* accumulated RSS usage in MB-usec */ + + /* Accumulated virtual memory usage in duration of a task. + * Same as acct_rss_mem1 above except that we keep track of VM usage. + */ + __u64 virtmem; /* accumulated VM usage in MB-usec */ + + /* High watermark of RSS usage in duration of a task, in KBytes. */ + __u64 hiwater_rss; /* High-watermark of RSS usage */ + + /* High watermark of VM usage in duration of a task, in KBytes. */ + __u64 hiwater_vm; /* High-water virtual memory usage */ + + /* The following four fields are I/O statistics of a task. */ + __u64 read_char; /* bytes read */ + __u64 write_char; /* bytes written */ + __u64 read_syscalls; /* read syscalls */ + __u64 write_syscalls; /* write syscalls */ + + /* Extended accounting fields end */ + +4) Per-task and per-thread statistics:: + + __u64 nvcsw; /* Context voluntary switch counter */ + __u64 nivcsw; /* Context involuntary switch counter */ + +5) Time accounting for SMT machines:: + + __u64 ac_utimescaled; /* utime scaled on frequency etc */ + __u64 ac_stimescaled; /* stime scaled on frequency etc */ + __u64 cpu_scaled_run_real_total; /* scaled cpu_run_real_total */ + +6) Extended delay accounting fields for memory reclaim:: + + /* Delay waiting for memory reclaim */ + __u64 freepages_count; + __u64 freepages_delay_total; + +:: + + } diff --git a/Documentation/accounting/taskstats.rst b/Documentation/accounting/taskstats.rst new file mode 100644 index 000000000..2a28b7f55 --- /dev/null +++ b/Documentation/accounting/taskstats.rst @@ -0,0 +1,180 @@ +============================= +Per-task statistics interface +============================= + + +Taskstats is a netlink-based interface for sending per-task and +per-process statistics from the kernel to userspace. + +Taskstats was designed for the following benefits: + +- efficiently provide statistics during lifetime of a task and on its exit +- unified interface for multiple accounting subsystems +- extensibility for use by future accounting patches + +Terminology +----------- + +"pid", "tid" and "task" are used interchangeably and refer to the standard +Linux task defined by struct task_struct. per-pid stats are the same as +per-task stats. + +"tgid", "process" and "thread group" are used interchangeably and refer to the +tasks that share an mm_struct i.e. the traditional Unix process. Despite the +use of tgid, there is no special treatment for the task that is thread group +leader - a process is deemed alive as long as it has any task belonging to it. + +Usage +----- + +To get statistics during a task's lifetime, userspace opens a unicast netlink +socket (NETLINK_GENERIC family) and sends commands specifying a pid or a tgid. +The response contains statistics for a task (if pid is specified) or the sum of +statistics for all tasks of the process (if tgid is specified). + +To obtain statistics for tasks which are exiting, the userspace listener +sends a register command and specifies a cpumask. Whenever a task exits on +one of the cpus in the cpumask, its per-pid statistics are sent to the +registered listener. Using cpumasks allows the data received by one listener +to be limited and assists in flow control over the netlink interface and is +explained in more detail below. + +If the exiting task is the last thread exiting its thread group, +an additional record containing the per-tgid stats is also sent to userspace. +The latter contains the sum of per-pid stats for all threads in the thread +group, both past and present. + +getdelays.c is a simple utility demonstrating usage of the taskstats interface +for reporting delay accounting statistics. Users can register cpumasks, +send commands and process responses, listen for per-tid/tgid exit data, +write the data received to a file and do basic flow control by increasing +receive buffer sizes. + +Interface +--------- + +The user-kernel interface is encapsulated in include/linux/taskstats.h + +To avoid this documentation becoming obsolete as the interface evolves, only +an outline of the current version is given. taskstats.h always overrides the +description here. + +struct taskstats is the common accounting structure for both per-pid and +per-tgid data. It is versioned and can be extended by each accounting subsystem +that is added to the kernel. The fields and their semantics are defined in the +taskstats.h file. + +The data exchanged between user and kernel space is a netlink message belonging +to the NETLINK_GENERIC family and using the netlink attributes interface. +The messages are in the format:: + + +----------+- - -+-------------+-------------------+ + | nlmsghdr | Pad | genlmsghdr | taskstats payload | + +----------+- - -+-------------+-------------------+ + + +The taskstats payload is one of the following three kinds: + +1. Commands: Sent from user to kernel. Commands to get data on +a pid/tgid consist of one attribute, of type TASKSTATS_CMD_ATTR_PID/TGID, +containing a u32 pid or tgid in the attribute payload. The pid/tgid denotes +the task/process for which userspace wants statistics. + +Commands to register/deregister interest in exit data from a set of cpus +consist of one attribute, of type +TASKSTATS_CMD_ATTR_REGISTER/DEREGISTER_CPUMASK and contain a cpumask in the +attribute payload. The cpumask is specified as an ascii string of +comma-separated cpu ranges e.g. to listen to exit data from cpus 1,2,3,5,7,8 +the cpumask would be "1-3,5,7-8". If userspace forgets to deregister interest +in cpus before closing the listening socket, the kernel cleans up its interest +set over time. However, for the sake of efficiency, an explicit deregistration +is advisable. + +2. Response for a command: sent from the kernel in response to a userspace +command. The payload is a series of three attributes of type: + +a) TASKSTATS_TYPE_AGGR_PID/TGID : attribute containing no payload but indicates +a pid/tgid will be followed by some stats. + +b) TASKSTATS_TYPE_PID/TGID: attribute whose payload is the pid/tgid whose stats +are being returned. + +c) TASKSTATS_TYPE_STATS: attribute with a struct taskstats as payload. The +same structure is used for both per-pid and per-tgid stats. + +3. New message sent by kernel whenever a task exits. The payload consists of a + series of attributes of the following type: + +a) TASKSTATS_TYPE_AGGR_PID: indicates next two attributes will be pid+stats +b) TASKSTATS_TYPE_PID: contains exiting task's pid +c) TASKSTATS_TYPE_STATS: contains the exiting task's per-pid stats +d) TASKSTATS_TYPE_AGGR_TGID: indicates next two attributes will be tgid+stats +e) TASKSTATS_TYPE_TGID: contains tgid of process to which task belongs +f) TASKSTATS_TYPE_STATS: contains the per-tgid stats for exiting task's process + + +per-tgid stats +-------------- + +Taskstats provides per-process stats, in addition to per-task stats, since +resource management is often done at a process granularity and aggregating task +stats in userspace alone is inefficient and potentially inaccurate (due to lack +of atomicity). + +However, maintaining per-process, in addition to per-task stats, within the +kernel has space and time overheads. To address this, the taskstats code +accumulates each exiting task's statistics into a process-wide data structure. +When the last task of a process exits, the process level data accumulated also +gets sent to userspace (along with the per-task data). + +When a user queries to get per-tgid data, the sum of all other live threads in +the group is added up and added to the accumulated total for previously exited +threads of the same thread group. + +Extending taskstats +------------------- + +There are two ways to extend the taskstats interface to export more +per-task/process stats as patches to collect them get added to the kernel +in future: + +1. Adding more fields to the end of the existing struct taskstats. Backward + compatibility is ensured by the version number within the + structure. Userspace will use only the fields of the struct that correspond + to the version its using. + +2. Defining separate statistic structs and using the netlink attributes + interface to return them. Since userspace processes each netlink attribute + independently, it can always ignore attributes whose type it does not + understand (because it is using an older version of the interface). + + +Choosing between 1. and 2. is a matter of trading off flexibility and +overhead. If only a few fields need to be added, then 1. is the preferable +path since the kernel and userspace don't need to incur the overhead of +processing new netlink attributes. But if the new fields expand the existing +struct too much, requiring disparate userspace accounting utilities to +unnecessarily receive large structures whose fields are of no interest, then +extending the attributes structure would be worthwhile. + +Flow control for taskstats +-------------------------- + +When the rate of task exits becomes large, a listener may not be able to keep +up with the kernel's rate of sending per-tid/tgid exit data leading to data +loss. This possibility gets compounded when the taskstats structure gets +extended and the number of cpus grows large. + +To avoid losing statistics, userspace should do one or more of the following: + +- increase the receive buffer sizes for the netlink sockets opened by + listeners to receive exit data. + +- create more listeners and reduce the number of cpus being listened to by + each listener. In the extreme case, there could be one listener for each cpu. + Users may also consider setting the cpu affinity of the listener to the subset + of cpus to which it listens, especially if they are listening to just one cpu. + +Despite these measures, if the userspace receives ENOBUFS error messages +indicated overflow of receive buffers, it should take measures to handle the +loss of data. |