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