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Diffstat (limited to 'Documentation/vm/numa_memory_policy.txt')
| -rw-r--r-- | Documentation/vm/numa_memory_policy.txt | 281 |
1 files changed, 201 insertions, 80 deletions
diff --git a/Documentation/vm/numa_memory_policy.txt b/Documentation/vm/numa_memory_policy.txt index dd49864..bad16d3 100644 --- a/Documentation/vm/numa_memory_policy.txt +++ b/Documentation/vm/numa_memory_policy.txt @@ -135,77 +135,58 @@ most general to most specific: Components of Memory Policies - A Linux memory policy is a tuple consisting of a "mode" and an optional set - of nodes. The mode determine the behavior of the policy, while the - optional set of nodes can be viewed as the arguments to the behavior. + A Linux memory policy consists of a "mode", optional mode flags, and an + optional set of nodes. The mode determines the behavior of the policy, + the optional mode flags determine the behavior of the mode, and the + optional set of nodes can be viewed as the arguments to the policy + behavior. Internally, memory policies are implemented by a reference counted structure, struct mempolicy. Details of this structure will be discussed in context, below, as required to explain the behavior. - Note: in some functions AND in the struct mempolicy itself, the mode - is called "policy". However, to avoid confusion with the policy tuple, - this document will continue to use the term "mode". - Linux memory policy supports the following 4 behavioral modes: - Default Mode--MPOL_DEFAULT: The behavior specified by this mode is - context or scope dependent. - - As mentioned in the Policy Scope section above, during normal - system operation, the System Default Policy is hard coded to - contain the Default mode. - - In this context, default mode means "local" allocation--that is - attempt to allocate the page from the node associated with the cpu - where the fault occurs. If the "local" node has no memory, or the - node's memory can be exhausted [no free pages available], local - allocation will "fallback to"--attempt to allocate pages from-- - "nearby" nodes, in order of increasing "distance". + Default Mode--MPOL_DEFAULT: This mode is only used in the memory + policy APIs. Internally, MPOL_DEFAULT is converted to the NULL + memory policy in all policy scopes. Any existing non-default policy + will simply be removed when MPOL_DEFAULT is specified. As a result, + MPOL_DEFAULT means "fall back to the next most specific policy scope." - Implementation detail -- subject to change: "Fallback" uses - a per node list of sibling nodes--called zonelists--built at - boot time, or when nodes or memory are added or removed from - the system [memory hotplug]. These per node zonelist are - constructed with nodes in order of increasing distance based - on information provided by the platform firmware. + For example, a NULL or default task policy will fall back to the + system default policy. A NULL or default vma policy will fall + back to the task policy. - When a task/process policy or a shared policy contains the Default - mode, this also means "local allocation", as described above. + When specified in one of the memory policy APIs, the Default mode + does not use the optional set of nodes. - In the context of a VMA, Default mode means "fall back to task - policy"--which may or may not specify Default mode. Thus, Default - mode can not be counted on to mean local allocation when used - on a non-shared region of the address space. However, see - MPOL_PREFERRED below. - - The Default mode does not use the optional set of nodes. + It is an error for the set of nodes specified for this policy to + be non-empty. MPOL_BIND: This mode specifies that memory must come from the - set of nodes specified by the policy. - - The memory policy APIs do not specify an order in which the nodes - will be searched. However, unlike "local allocation", the Bind - policy does not consider the distance between the nodes. Rather, - allocations will fallback to the nodes specified by the policy in - order of numeric node id. Like everything in Linux, this is subject - to change. + set of nodes specified by the policy. Memory will be allocated from + the node in the set with sufficient free memory that is closest to + the node where the allocation takes place. MPOL_PREFERRED: This mode specifies that the allocation should be attempted from the single node specified in the policy. If that - allocation fails, the kernel will search other nodes, exactly as - it would for a local allocation that started at the preferred node - in increasing distance from the preferred node. "Local" allocation - policy can be viewed as a Preferred policy that starts at the node + allocation fails, the kernel will search other nodes, in order of + increasing distance from the preferred node based on information + provided by the platform firmware. containing the cpu where the allocation takes place. Internally, the Preferred policy uses a single node--the - preferred_node member of struct mempolicy. A "distinguished - value of this preferred_node, currently '-1', is interpreted - as "the node containing the cpu where the allocation takes - place"--local allocation. This is the way to specify - local allocation for a specific range of addresses--i.e. for - VMA policies. + preferred_node member of struct mempolicy. When the internal + mode flag MPOL_F_LOCAL is set, the preferred_node is ignored and + the policy is interpreted as local allocation. "Local" allocation + policy can be viewed as a Preferred policy that starts at the node + containing the cpu where the allocation takes place. + + It is possible for the user to specify that local allocation is + always preferred by passing an empty nodemask with this mode. + If an empty nodemask is passed, the policy cannot use the + MPOL_F_STATIC_NODES or MPOL_F_RELATIVE_NODES flags described + below. MPOL_INTERLEAVED: This mode specifies that page allocations be interleaved, on a page granularity, across the nodes specified in @@ -231,6 +212,154 @@ Components of Memory Policies the temporary interleaved system default policy works in this mode. + Linux memory policy supports the following optional mode flags: + + MPOL_F_STATIC_NODES: This flag specifies that the nodemask passed by + the user should not be remapped if the task or VMA's set of allowed + nodes changes after the memory policy has been defined. + + Without this flag, anytime a mempolicy is rebound because of a + change in the set of allowed nodes, the node (Preferred) or + nodemask (Bind, Interleave) is remapped to the new set of + allowed nodes. This may result in nodes being used that were + previously undesired. + + With this flag, if the user-specified nodes overlap with the + nodes allowed by the task's cpuset, then the memory policy is + applied to their intersection. If the two sets of nodes do not + overlap, the Default policy is used. + + For example, consider a task that is attached to a cpuset with + mems 1-3 that sets an Interleave policy over the same set. If + the cpuset's mems change to 3-5, the Interleave will now occur + over nodes 3, 4, and 5. With this flag, however, since only node + 3 is allowed from the user's nodemask, the "interleave" only + occurs over that node. If no nodes from the user's nodemask are + now allowed, the Default behavior is used. + + MPOL_F_STATIC_NODES cannot be combined with the + MPOL_F_RELATIVE_NODES flag. It also cannot be used for + MPOL_PREFERRED policies that were created with an empty nodemask + (local allocation). + + MPOL_F_RELATIVE_NODES: This flag specifies that the nodemask passed + by the user will be mapped relative to the set of the task or VMA's + set of allowed nodes. The kernel stores the user-passed nodemask, + and if the allowed nodes changes, then that original nodemask will + be remapped relative to the new set of allowed nodes. + + Without this flag (and without MPOL_F_STATIC_NODES), anytime a + mempolicy is rebound because of a change in the set of allowed + nodes, the node (Preferred) or nodemask (Bind, Interleave) is + remapped to the new set of allowed nodes. That remap may not + preserve the relative nature of the user's passed nodemask to its + set of allowed nodes upon successive rebinds: a nodemask of + 1,3,5 may be remapped to 7-9 and then to 1-3 if the set of + allowed nodes is restored to its original state. + + With this flag, the remap is done so that the node numbers from + the user's passed nodemask are relative to the set of allowed + nodes. In other words, if nodes 0, 2, and 4 are set in the user's + nodemask, the policy will be effected over the first (and in the + Bind or Interleave case, the third and fifth) nodes in the set of + allowed nodes. The nodemask passed by the user represents nodes + relative to task or VMA's set of allowed nodes. + + If the user's nodemask includes nodes that are outside the range + of the new set of allowed nodes (for example, node 5 is set in + the user's nodemask when the set of allowed nodes is only 0-3), + then the remap wraps around to the beginning of the nodemask and, + if not already set, sets the node in the mempolicy nodemask. + + For example, consider a task that is attached to a cpuset with + mems 2-5 that sets an Interleave policy over the same set with + MPOL_F_RELATIVE_NODES. If the cpuset's mems change to 3-7, the + interleave now occurs over nodes 3,5-6. If the cpuset's mems + then change to 0,2-3,5, then the interleave occurs over nodes + 0,3,5. + + Thanks to the consistent remapping, applications preparing + nodemasks to specify memory policies using this flag should + disregard their current, actual cpuset imposed memory placement + and prepare the nodemask as if they were always located on + memory nodes 0 to N-1, where N is the number of memory nodes the + policy is intended to manage. Let the kernel then remap to the + set of memory nodes allowed by the task's cpuset, as that may + change over time. + + MPOL_F_RELATIVE_NODES cannot be combined with the + MPOL_F_STATIC_NODES flag. It also cannot be used for + MPOL_PREFERRED policies that were created with an empty nodemask + (local allocation). + +MEMORY POLICY REFERENCE COUNTING + +To resolve use/free races, struct mempolicy contains an atomic reference +count field. Internal interfaces, mpol_get()/mpol_put() increment and +decrement this reference count, respectively. mpol_put() will only free +the structure back to the mempolicy kmem cache when the reference count +goes to zero. + +When a new memory policy is allocated, it's reference count is initialized +to '1', representing the reference held by the task that is installing the +new policy. When a pointer to a memory policy structure is stored in another +structure, another reference is added, as the task's reference will be dropped +on completion of the policy installation. + +During run-time "usage" of the policy, we attempt to minimize atomic operations +on the reference count, as this can lead to cache lines bouncing between cpus +and NUMA nodes. "Usage" here means one of the following: + +1) querying of the policy, either by the task itself [using the get_mempolicy() + API discussed below] or by another task using the /proc/<pid>/numa_maps + interface. + +2) examination of the policy to determine the policy mode and associated node + or node lists, if any, for page allocation. This is considered a "hot + path". Note that for MPOL_BIND, the "usage" extends across the entire + allocation process, which may sleep during page reclaimation, because the + BIND policy nodemask is used, by reference, to filter ineligible nodes. + +We can avoid taking an extra reference during the usages listed above as +follows: + +1) we never need to get/free the system default policy as this is never + changed nor freed, once the system is up and running. + +2) for querying the policy, we do not need to take an extra reference on the + target task's task policy nor vma policies because we always acquire the + task's mm's mmap_sem for read during the query. The set_mempolicy() and + mbind() APIs [see below] always acquire the mmap_sem for write when + installing or replacing task or vma policies. Thus, there is no possibility + of a task or thread freeing a policy while another task or thread is + querying it. + +3) Page allocation usage of task or vma policy occurs in the fault path where + we hold them mmap_sem for read. Again, because replacing the task or vma + policy requires that the mmap_sem be held for write, the policy can't be + freed out from under us while we're using it for page allocation. + +4) Shared policies require special consideration. One task can replace a + shared memory policy while another task, with a distinct mmap_sem, is + querying or allocating a page based on the policy. To resolve this + potential race, the shared policy infrastructure adds an extra reference + to the shared policy during lookup while holding a spin lock on the shared + policy management structure. This requires that we drop this extra + reference when we're finished "using" the policy. We must drop the + extra reference on shared policies in the same query/allocation paths + used for non-shared policies. For this reason, shared policies are marked + as such, and the extra reference is dropped "conditionally"--i.e., only + for shared policies. + + Because of this extra reference counting, and because we must lookup + shared policies in a tree structure under spinlock, shared policies are + more expensive to use in the page allocation path. This is expecially + true for shared policies on shared memory regions shared by tasks running + on different NUMA nodes. This extra overhead can be avoided by always + falling back to task or system default policy for shared memory regions, + or by prefaulting the entire shared memory region into memory and locking + it down. However, this might not be appropriate for all applications. + MEMORY POLICY APIs Linux supports 3 system calls for controlling memory policy. These APIS @@ -251,7 +380,9 @@ Set [Task] Memory Policy: Set's the calling task's "task/process memory policy" to mode specified by the 'mode' argument and the set of nodes defined by 'nmask'. 'nmask' points to a bit mask of node ids containing - at least 'maxnode' ids. + at least 'maxnode' ids. Optional mode flags may be passed by + combining the 'mode' argument with the flag (for example: + MPOL_INTERLEAVE | MPOL_F_STATIC_NODES). See the set_mempolicy(2) man page for more details @@ -303,29 +434,19 @@ MEMORY POLICIES AND CPUSETS Memory policies work within cpusets as described above. For memory policies that require a node or set of nodes, the nodes are restricted to the set of nodes whose memories are allowed by the cpuset constraints. If the nodemask -specified for the policy contains nodes that are not allowed by the cpuset, or -the intersection of the set of nodes specified for the policy and the set of -nodes with memory is the empty set, the policy is considered invalid -and cannot be installed. - -The interaction of memory policies and cpusets can be problematic for a -couple of reasons: - -1) the memory policy APIs take physical node id's as arguments. As mentioned - above, it is illegal to specify nodes that are not allowed in the cpuset. - The application must query the allowed nodes using the get_mempolicy() - API with the MPOL_F_MEMS_ALLOWED flag to determine the allowed nodes and - restrict itself to those nodes. However, the resources available to a - cpuset can be changed by the system administrator, or a workload manager - application, at any time. So, a task may still get errors attempting to - specify policy nodes, and must query the allowed memories again. - -2) when tasks in two cpusets share access to a memory region, such as shared - memory segments created by shmget() of mmap() with the MAP_ANONYMOUS and - MAP_SHARED flags, and any of the tasks install shared policy on the region, - only nodes whose memories are allowed in both cpusets may be used in the - policies. Obtaining this information requires "stepping outside" the - memory policy APIs to use the cpuset information and requires that one - know in what cpusets other task might be attaching to the shared region. - Furthermore, if the cpusets' allowed memory sets are disjoint, "local" - allocation is the only valid policy. +specified for the policy contains nodes that are not allowed by the cpuset and +MPOL_F_RELATIVE_NODES is not used, the intersection of the set of nodes +specified for the policy and the set of nodes with memory is used. If the +result is the empty set, the policy is considered invalid and cannot be +installed. If MPOL_F_RELATIVE_NODES is used, the policy's nodes are mapped +onto and folded into the task's set of allowed nodes as previously described. + +The interaction of memory policies and cpusets can be problematic when tasks +in two cpusets share access to a memory region, such as shared memory segments +created by shmget() of mmap() with the MAP_ANONYMOUS and MAP_SHARED flags, and +any of the tasks install shared policy on the region, only nodes whose +memories are allowed in both cpusets may be used in the policies. Obtaining +this information requires "stepping outside" the memory policy APIs to use the +cpuset information and requires that one know in what cpusets other task might +be attaching to the shared region. Furthermore, if the cpusets' allowed +memory sets are disjoint, "local" allocation is the only valid policy. |