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diff --git a/Documentation/arm/cluster-pm-race-avoidance.rst b/Documentation/arm/cluster-pm-race-avoidance.rst new file mode 100644 index 000000000..aa58603d3 --- /dev/null +++ b/Documentation/arm/cluster-pm-race-avoidance.rst @@ -0,0 +1,533 @@ +========================================================= +Cluster-wide Power-up/power-down race avoidance algorithm +========================================================= + +This file documents the algorithm which is used to coordinate CPU and +cluster setup and teardown operations and to manage hardware coherency +controls safely. + +The section "Rationale" explains what the algorithm is for and why it is +needed. "Basic model" explains general concepts using a simplified view +of the system. The other sections explain the actual details of the +algorithm in use. + + +Rationale +--------- + +In a system containing multiple CPUs, it is desirable to have the +ability to turn off individual CPUs when the system is idle, reducing +power consumption and thermal dissipation. + +In a system containing multiple clusters of CPUs, it is also desirable +to have the ability to turn off entire clusters. + +Turning entire clusters off and on is a risky business, because it +involves performing potentially destructive operations affecting a group +of independently running CPUs, while the OS continues to run. This +means that we need some coordination in order to ensure that critical +cluster-level operations are only performed when it is truly safe to do +so. + +Simple locking may not be sufficient to solve this problem, because +mechanisms like Linux spinlocks may rely on coherency mechanisms which +are not immediately enabled when a cluster powers up. Since enabling or +disabling those mechanisms may itself be a non-atomic operation (such as +writing some hardware registers and invalidating large caches), other +methods of coordination are required in order to guarantee safe +power-down and power-up at the cluster level. + +The mechanism presented in this document describes a coherent memory +based protocol for performing the needed coordination. It aims to be as +lightweight as possible, while providing the required safety properties. + + +Basic model +----------- + +Each cluster and CPU is assigned a state, as follows: + + - DOWN + - COMING_UP + - UP + - GOING_DOWN + +:: + + +---------> UP ----------+ + | v + + COMING_UP GOING_DOWN + + ^ | + +--------- DOWN <--------+ + + +DOWN: + The CPU or cluster is not coherent, and is either powered off or + suspended, or is ready to be powered off or suspended. + +COMING_UP: + The CPU or cluster has committed to moving to the UP state. + It may be part way through the process of initialisation and + enabling coherency. + +UP: + The CPU or cluster is active and coherent at the hardware + level. A CPU in this state is not necessarily being used + actively by the kernel. + +GOING_DOWN: + The CPU or cluster has committed to moving to the DOWN + state. It may be part way through the process of teardown and + coherency exit. + + +Each CPU has one of these states assigned to it at any point in time. +The CPU states are described in the "CPU state" section, below. + +Each cluster is also assigned a state, but it is necessary to split the +state value into two parts (the "cluster" state and "inbound" state) and +to introduce additional states in order to avoid races between different +CPUs in the cluster simultaneously modifying the state. The cluster- +level states are described in the "Cluster state" section. + +To help distinguish the CPU states from cluster states in this +discussion, the state names are given a `CPU_` prefix for the CPU states, +and a `CLUSTER_` or `INBOUND_` prefix for the cluster states. + + +CPU state +--------- + +In this algorithm, each individual core in a multi-core processor is +referred to as a "CPU". CPUs are assumed to be single-threaded: +therefore, a CPU can only be doing one thing at a single point in time. + +This means that CPUs fit the basic model closely. + +The algorithm defines the following states for each CPU in the system: + + - CPU_DOWN + - CPU_COMING_UP + - CPU_UP + - CPU_GOING_DOWN + +:: + + cluster setup and + CPU setup complete policy decision + +-----------> CPU_UP ------------+ + | v + + CPU_COMING_UP CPU_GOING_DOWN + + ^ | + +----------- CPU_DOWN <----------+ + policy decision CPU teardown complete + or hardware event + + +The definitions of the four states correspond closely to the states of +the basic model. + +Transitions between states occur as follows. + +A trigger event (spontaneous) means that the CPU can transition to the +next state as a result of making local progress only, with no +requirement for any external event to happen. + + +CPU_DOWN: + A CPU reaches the CPU_DOWN state when it is ready for + power-down. On reaching this state, the CPU will typically + power itself down or suspend itself, via a WFI instruction or a + firmware call. + + Next state: + CPU_COMING_UP + Conditions: + none + + Trigger events: + a) an explicit hardware power-up operation, resulting + from a policy decision on another CPU; + + b) a hardware event, such as an interrupt. + + +CPU_COMING_UP: + A CPU cannot start participating in hardware coherency until the + cluster is set up and coherent. If the cluster is not ready, + then the CPU will wait in the CPU_COMING_UP state until the + cluster has been set up. + + Next state: + CPU_UP + Conditions: + The CPU's parent cluster must be in CLUSTER_UP. + Trigger events: + Transition of the parent cluster to CLUSTER_UP. + + Refer to the "Cluster state" section for a description of the + CLUSTER_UP state. + + +CPU_UP: + When a CPU reaches the CPU_UP state, it is safe for the CPU to + start participating in local coherency. + + This is done by jumping to the kernel's CPU resume code. + + Note that the definition of this state is slightly different + from the basic model definition: CPU_UP does not mean that the + CPU is coherent yet, but it does mean that it is safe to resume + the kernel. The kernel handles the rest of the resume + procedure, so the remaining steps are not visible as part of the + race avoidance algorithm. + + The CPU remains in this state until an explicit policy decision + is made to shut down or suspend the CPU. + + Next state: + CPU_GOING_DOWN + Conditions: + none + Trigger events: + explicit policy decision + + +CPU_GOING_DOWN: + While in this state, the CPU exits coherency, including any + operations required to achieve this (such as cleaning data + caches). + + Next state: + CPU_DOWN + Conditions: + local CPU teardown complete + Trigger events: + (spontaneous) + + +Cluster state +------------- + +A cluster is a group of connected CPUs with some common resources. +Because a cluster contains multiple CPUs, it can be doing multiple +things at the same time. This has some implications. In particular, a +CPU can start up while another CPU is tearing the cluster down. + +In this discussion, the "outbound side" is the view of the cluster state +as seen by a CPU tearing the cluster down. The "inbound side" is the +view of the cluster state as seen by a CPU setting the CPU up. + +In order to enable safe coordination in such situations, it is important +that a CPU which is setting up the cluster can advertise its state +independently of the CPU which is tearing down the cluster. For this +reason, the cluster state is split into two parts: + + "cluster" state: The global state of the cluster; or the state + on the outbound side: + + - CLUSTER_DOWN + - CLUSTER_UP + - CLUSTER_GOING_DOWN + + "inbound" state: The state of the cluster on the inbound side. + + - INBOUND_NOT_COMING_UP + - INBOUND_COMING_UP + + + The different pairings of these states results in six possible + states for the cluster as a whole:: + + CLUSTER_UP + +==========> INBOUND_NOT_COMING_UP -------------+ + # | + | + CLUSTER_UP <----+ | + INBOUND_COMING_UP | v + + ^ CLUSTER_GOING_DOWN CLUSTER_GOING_DOWN + # INBOUND_COMING_UP <=== INBOUND_NOT_COMING_UP + + CLUSTER_DOWN | | + INBOUND_COMING_UP <----+ | + | + ^ | + +=========== CLUSTER_DOWN <------------+ + INBOUND_NOT_COMING_UP + + Transitions -----> can only be made by the outbound CPU, and + only involve changes to the "cluster" state. + + Transitions ===##> can only be made by the inbound CPU, and only + involve changes to the "inbound" state, except where there is no + further transition possible on the outbound side (i.e., the + outbound CPU has put the cluster into the CLUSTER_DOWN state). + + The race avoidance algorithm does not provide a way to determine + which exact CPUs within the cluster play these roles. This must + be decided in advance by some other means. Refer to the section + "Last man and first man selection" for more explanation. + + + CLUSTER_DOWN/INBOUND_NOT_COMING_UP is the only state where the + cluster can actually be powered down. + + The parallelism of the inbound and outbound CPUs is observed by + the existence of two different paths from CLUSTER_GOING_DOWN/ + INBOUND_NOT_COMING_UP (corresponding to GOING_DOWN in the basic + model) to CLUSTER_DOWN/INBOUND_COMING_UP (corresponding to + COMING_UP in the basic model). The second path avoids cluster + teardown completely. + + CLUSTER_UP/INBOUND_COMING_UP is equivalent to UP in the basic + model. The final transition to CLUSTER_UP/INBOUND_NOT_COMING_UP + is trivial and merely resets the state machine ready for the + next cycle. + + Details of the allowable transitions follow. + + The next state in each case is notated + + <cluster state>/<inbound state> (<transitioner>) + + where the <transitioner> is the side on which the transition + can occur; either the inbound or the outbound side. + + +CLUSTER_DOWN/INBOUND_NOT_COMING_UP: + Next state: + CLUSTER_DOWN/INBOUND_COMING_UP (inbound) + Conditions: + none + + Trigger events: + a) an explicit hardware power-up operation, resulting + from a policy decision on another CPU; + + b) a hardware event, such as an interrupt. + + +CLUSTER_DOWN/INBOUND_COMING_UP: + + In this state, an inbound CPU sets up the cluster, including + enabling of hardware coherency at the cluster level and any + other operations (such as cache invalidation) which are required + in order to achieve this. + + The purpose of this state is to do sufficient cluster-level + setup to enable other CPUs in the cluster to enter coherency + safely. + + Next state: + CLUSTER_UP/INBOUND_COMING_UP (inbound) + Conditions: + cluster-level setup and hardware coherency complete + Trigger events: + (spontaneous) + + +CLUSTER_UP/INBOUND_COMING_UP: + + Cluster-level setup is complete and hardware coherency is + enabled for the cluster. Other CPUs in the cluster can safely + enter coherency. + + This is a transient state, leading immediately to + CLUSTER_UP/INBOUND_NOT_COMING_UP. All other CPUs on the cluster + should consider treat these two states as equivalent. + + Next state: + CLUSTER_UP/INBOUND_NOT_COMING_UP (inbound) + Conditions: + none + Trigger events: + (spontaneous) + + +CLUSTER_UP/INBOUND_NOT_COMING_UP: + + Cluster-level setup is complete and hardware coherency is + enabled for the cluster. Other CPUs in the cluster can safely + enter coherency. + + The cluster will remain in this state until a policy decision is + made to power the cluster down. + + Next state: + CLUSTER_GOING_DOWN/INBOUND_NOT_COMING_UP (outbound) + Conditions: + none + Trigger events: + policy decision to power down the cluster + + +CLUSTER_GOING_DOWN/INBOUND_NOT_COMING_UP: + + An outbound CPU is tearing the cluster down. The selected CPU + must wait in this state until all CPUs in the cluster are in the + CPU_DOWN state. + + When all CPUs are in the CPU_DOWN state, the cluster can be torn + down, for example by cleaning data caches and exiting + cluster-level coherency. + + To avoid wasteful unnecessary teardown operations, the outbound + should check the inbound cluster state for asynchronous + transitions to INBOUND_COMING_UP. Alternatively, individual + CPUs can be checked for entry into CPU_COMING_UP or CPU_UP. + + + Next states: + + CLUSTER_DOWN/INBOUND_NOT_COMING_UP (outbound) + Conditions: + cluster torn down and ready to power off + Trigger events: + (spontaneous) + + CLUSTER_GOING_DOWN/INBOUND_COMING_UP (inbound) + Conditions: + none + + Trigger events: + a) an explicit hardware power-up operation, + resulting from a policy decision on another + CPU; + + b) a hardware event, such as an interrupt. + + +CLUSTER_GOING_DOWN/INBOUND_COMING_UP: + + The cluster is (or was) being torn down, but another CPU has + come online in the meantime and is trying to set up the cluster + again. + + If the outbound CPU observes this state, it has two choices: + + a) back out of teardown, restoring the cluster to the + CLUSTER_UP state; + + b) finish tearing the cluster down and put the cluster + in the CLUSTER_DOWN state; the inbound CPU will + set up the cluster again from there. + + Choice (a) permits the removal of some latency by avoiding + unnecessary teardown and setup operations in situations where + the cluster is not really going to be powered down. + + + Next states: + + CLUSTER_UP/INBOUND_COMING_UP (outbound) + Conditions: + cluster-level setup and hardware + coherency complete + + Trigger events: + (spontaneous) + + CLUSTER_DOWN/INBOUND_COMING_UP (outbound) + Conditions: + cluster torn down and ready to power off + + Trigger events: + (spontaneous) + + +Last man and First man selection +-------------------------------- + +The CPU which performs cluster tear-down operations on the outbound side +is commonly referred to as the "last man". + +The CPU which performs cluster setup on the inbound side is commonly +referred to as the "first man". + +The race avoidance algorithm documented above does not provide a +mechanism to choose which CPUs should play these roles. + + +Last man: + +When shutting down the cluster, all the CPUs involved are initially +executing Linux and hence coherent. Therefore, ordinary spinlocks can +be used to select a last man safely, before the CPUs become +non-coherent. + + +First man: + +Because CPUs may power up asynchronously in response to external wake-up +events, a dynamic mechanism is needed to make sure that only one CPU +attempts to play the first man role and do the cluster-level +initialisation: any other CPUs must wait for this to complete before +proceeding. + +Cluster-level initialisation may involve actions such as configuring +coherency controls in the bus fabric. + +The current implementation in mcpm_head.S uses a separate mutual exclusion +mechanism to do this arbitration. This mechanism is documented in +detail in vlocks.txt. + + +Features and Limitations +------------------------ + +Implementation: + + The current ARM-based implementation is split between + arch/arm/common/mcpm_head.S (low-level inbound CPU operations) and + arch/arm/common/mcpm_entry.c (everything else): + + __mcpm_cpu_going_down() signals the transition of a CPU to the + CPU_GOING_DOWN state. + + __mcpm_cpu_down() signals the transition of a CPU to the CPU_DOWN + state. + + A CPU transitions to CPU_COMING_UP and then to CPU_UP via the + low-level power-up code in mcpm_head.S. This could + involve CPU-specific setup code, but in the current + implementation it does not. + + __mcpm_outbound_enter_critical() and __mcpm_outbound_leave_critical() + handle transitions from CLUSTER_UP to CLUSTER_GOING_DOWN + and from there to CLUSTER_DOWN or back to CLUSTER_UP (in + the case of an aborted cluster power-down). + + These functions are more complex than the __mcpm_cpu_*() + functions due to the extra inter-CPU coordination which + is needed for safe transitions at the cluster level. + + A cluster transitions from CLUSTER_DOWN back to CLUSTER_UP via + the low-level power-up code in mcpm_head.S. This + typically involves platform-specific setup code, + provided by the platform-specific power_up_setup + function registered via mcpm_sync_init. + +Deep topologies: + + As currently described and implemented, the algorithm does not + support CPU topologies involving more than two levels (i.e., + clusters of clusters are not supported). The algorithm could be + extended by replicating the cluster-level states for the + additional topological levels, and modifying the transition + rules for the intermediate (non-outermost) cluster levels. + + +Colophon +-------- + +Originally created and documented by Dave Martin for Linaro Limited, in +collaboration with Nicolas Pitre and Achin Gupta. + +Copyright (C) 2012-2013 Linaro Limited +Distributed under the terms of Version 2 of the GNU General Public +License, as defined in linux/COPYING. |