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author | 2018-05-08 10:02:10 +0300 | |
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committer | 2018-05-08 09:31:31 -0600 | |
commit | 3ecf53e41a642d4172cff1f641b23fa1baaa229a (patch) | |
tree | 346ca56d48a44cfa9f69df67f4b2c28afc7f0bb8 /Documentation/vm | |
parent | docs/vm: numa_memory_policy: s/Linux memory policy/NUMA memory policy/ (diff) | |
download | linux-3ecf53e41a642d4172cff1f641b23fa1baaa229a.tar.gz linux-3ecf53e41a642d4172cff1f641b23fa1baaa229a.tar.bz2 linux-3ecf53e41a642d4172cff1f641b23fa1baaa229a.zip |
docs/vm: move numa_memory_policy.rst to Documentation/admin-guide/mm
The document describes userspace API and as such it belongs to
Documentation/admin-guide/mm
Signed-off-by: Mike Rapoport <rppt@linux.vnet.ibm.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Diffstat (limited to 'Documentation/vm')
-rw-r--r-- | Documentation/vm/00-INDEX | 2 | ||||
-rw-r--r-- | Documentation/vm/index.rst | 1 | ||||
-rw-r--r-- | Documentation/vm/numa.rst | 2 | ||||
-rw-r--r-- | Documentation/vm/numa_memory_policy.rst | 495 |
4 files changed, 1 insertions, 499 deletions
diff --git a/Documentation/vm/00-INDEX b/Documentation/vm/00-INDEX index f8a96ca16b7a..f4a4f3e884cf 100644 --- a/Documentation/vm/00-INDEX +++ b/Documentation/vm/00-INDEX @@ -22,8 +22,6 @@ mmu_notifier.rst - a note about clearing pte/pmd and mmu notifications numa.rst - information about NUMA specific code in the Linux vm. -numa_memory_policy.rst - - documentation of concepts and APIs of the 2.6 memory policy support. overcommit-accounting.rst - description of the Linux kernels overcommit handling modes. page_frags.rst diff --git a/Documentation/vm/index.rst b/Documentation/vm/index.rst index ed58cb9f9675..8e1cc667eef1 100644 --- a/Documentation/vm/index.rst +++ b/Documentation/vm/index.rst @@ -14,7 +14,6 @@ various features of the Linux memory management :maxdepth: 1 ksm - numa_memory_policy transhuge swap_numa zswap diff --git a/Documentation/vm/numa.rst b/Documentation/vm/numa.rst index aada84bc8c46..185d8a568168 100644 --- a/Documentation/vm/numa.rst +++ b/Documentation/vm/numa.rst @@ -110,7 +110,7 @@ to improve NUMA locality using various CPU affinity command line interfaces, such as taskset(1) and numactl(1), and program interfaces such as sched_setaffinity(2). Further, one can modify the kernel's default local allocation behavior using Linux NUMA memory policy. -[see Documentation/vm/numa_memory_policy.rst.] +[see Documentation/admin-guide/mm/numa_memory_policy.rst.] System administrators can restrict the CPUs and nodes' memories that a non- privileged user can specify in the scheduling or NUMA commands and functions diff --git a/Documentation/vm/numa_memory_policy.rst b/Documentation/vm/numa_memory_policy.rst deleted file mode 100644 index d78c5b315f72..000000000000 --- a/Documentation/vm/numa_memory_policy.rst +++ /dev/null @@ -1,495 +0,0 @@ -.. _numa_memory_policy: - -================== -NUMA Memory Policy -================== - -What is NUMA Memory Policy? -============================ - -In the Linux kernel, "memory policy" determines from which node the kernel will -allocate memory in a NUMA system or in an emulated NUMA system. Linux has -supported platforms with Non-Uniform Memory Access architectures since 2.4.?. -The current memory policy support was added to Linux 2.6 around May 2004. This -document attempts to describe the concepts and APIs of the 2.6 memory policy -support. - -Memory policies should not be confused with cpusets -(``Documentation/cgroup-v1/cpusets.txt``) -which is an administrative mechanism for restricting the nodes from which -memory may be allocated by a set of processes. Memory policies are a -programming interface that a NUMA-aware application can take advantage of. When -both cpusets and policies are applied to a task, the restrictions of the cpuset -takes priority. See :ref:`Memory Policies and cpusets <mem_pol_and_cpusets>` -below for more details. - -Memory Policy Concepts -====================== - -Scope of Memory Policies ------------------------- - -The Linux kernel supports _scopes_ of memory policy, described here from -most general to most specific: - -System Default Policy - this policy is "hard coded" into the kernel. It is the policy - that governs all page allocations that aren't controlled by - one of the more specific policy scopes discussed below. When - the system is "up and running", the system default policy will - use "local allocation" described below. However, during boot - up, the system default policy will be set to interleave - allocations across all nodes with "sufficient" memory, so as - not to overload the initial boot node with boot-time - allocations. - -Task/Process Policy - this is an optional, per-task policy. When defined for a - specific task, this policy controls all page allocations made - by or on behalf of the task that aren't controlled by a more - specific scope. If a task does not define a task policy, then - all page allocations that would have been controlled by the - task policy "fall back" to the System Default Policy. - - The task policy applies to the entire address space of a task. Thus, - it is inheritable, and indeed is inherited, across both fork() - [clone() w/o the CLONE_VM flag] and exec*(). This allows a parent task - to establish the task policy for a child task exec()'d from an - executable image that has no awareness of memory policy. See the - :ref:`Memory Policy APIs <memory_policy_apis>` section, - below, for an overview of the system call - that a task may use to set/change its task/process policy. - - In a multi-threaded task, task policies apply only to the thread - [Linux kernel task] that installs the policy and any threads - subsequently created by that thread. Any sibling threads existing - at the time a new task policy is installed retain their current - policy. - - A task policy applies only to pages allocated after the policy is - installed. Any pages already faulted in by the task when the task - changes its task policy remain where they were allocated based on - the policy at the time they were allocated. - -.. _vma_policy: - -VMA Policy - A "VMA" or "Virtual Memory Area" refers to a range of a task's - virtual address space. A task may define a specific policy for a range - of its virtual address space. See the - :ref:`Memory Policy APIs <memory_policy_apis>` section, - below, for an overview of the mbind() system call used to set a VMA - policy. - - A VMA policy will govern the allocation of pages that back - this region of the address space. Any regions of the task's - address space that don't have an explicit VMA policy will fall - back to the task policy, which may itself fall back to the - System Default Policy. - - VMA policies have a few complicating details: - - * VMA policy applies ONLY to anonymous pages. These include - pages allocated for anonymous segments, such as the task - stack and heap, and any regions of the address space - mmap()ed with the MAP_ANONYMOUS flag. If a VMA policy is - applied to a file mapping, it will be ignored if the mapping - used the MAP_SHARED flag. If the file mapping used the - MAP_PRIVATE flag, the VMA policy will only be applied when - an anonymous page is allocated on an attempt to write to the - mapping-- i.e., at Copy-On-Write. - - * VMA policies are shared between all tasks that share a - virtual address space--a.k.a. threads--independent of when - the policy is installed; and they are inherited across - fork(). However, because VMA policies refer to a specific - region of a task's address space, and because the address - space is discarded and recreated on exec*(), VMA policies - are NOT inheritable across exec(). Thus, only NUMA-aware - applications may use VMA policies. - - * A task may install a new VMA policy on a sub-range of a - previously mmap()ed region. When this happens, Linux splits - the existing virtual memory area into 2 or 3 VMAs, each with - it's own policy. - - * By default, VMA policy applies only to pages allocated after - the policy is installed. Any pages already faulted into the - VMA range remain where they were allocated based on the - policy at the time they were allocated. However, since - 2.6.16, Linux supports page migration via the mbind() system - call, so that page contents can be moved to match a newly - installed policy. - -Shared Policy - Conceptually, shared policies apply to "memory objects" mapped - shared into one or more tasks' distinct address spaces. An - application installs shared policies the same way as VMA - policies--using the mbind() system call specifying a range of - virtual addresses that map the shared object. However, unlike - VMA policies, which can be considered to be an attribute of a - range of a task's address space, shared policies apply - directly to the shared object. Thus, all tasks that attach to - the object share the policy, and all pages allocated for the - shared object, by any task, will obey the shared policy. - - As of 2.6.22, only shared memory segments, created by shmget() or - mmap(MAP_ANONYMOUS|MAP_SHARED), support shared policy. When shared - policy support was added to Linux, the associated data structures were - added to hugetlbfs shmem segments. At the time, hugetlbfs did not - support allocation at fault time--a.k.a lazy allocation--so hugetlbfs - shmem segments were never "hooked up" to the shared policy support. - Although hugetlbfs segments now support lazy allocation, their support - for shared policy has not been completed. - - As mentioned above in :ref:`VMA policies <vma_policy>` section, - allocations of page cache pages for regular files mmap()ed - with MAP_SHARED ignore any VMA policy installed on the virtual - address range backed by the shared file mapping. Rather, - shared page cache pages, including pages backing private - mappings that have not yet been written by the task, follow - task policy, if any, else System Default Policy. - - The shared policy infrastructure supports different policies on subset - ranges of the shared object. However, Linux still splits the VMA of - the task that installs the policy for each range of distinct policy. - Thus, different tasks that attach to a shared memory segment can have - different VMA configurations mapping that one shared object. This - can be seen by examining the /proc/<pid>/numa_maps of tasks sharing - a shared memory region, when one task has installed shared policy on - one or more ranges of the region. - -Components of Memory Policies ------------------------------ - -A NUMA 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. - -NUMA memory policy supports the following 4 behavioral modes: - -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." - - 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 specified in one of the memory policy APIs, 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. 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, in order - of increasing distance from the preferred node based on - information provided by the platform firmware. - - Internally, the Preferred policy uses a single node--the - 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 the policy. - This mode also behaves slightly differently, based on the - context where it is used: - - For allocation of anonymous pages and shared memory pages, - Interleave mode indexes the set of nodes specified by the - policy using the page offset of the faulting address into the - segment [VMA] containing the address modulo the number of - nodes specified by the policy. It then attempts to allocate a - page, starting at the selected node, as if the node had been - specified by a Preferred policy or had been selected by a - local allocation. That is, allocation will follow the per - node zonelist. - - For allocation of page cache pages, Interleave mode indexes - the set of nodes specified by the policy using a node counter - maintained per task. This counter wraps around to the lowest - specified node after it reaches the highest specified node. - This will tend to spread the pages out over the nodes - specified by the policy based on the order in which they are - allocated, rather than based on any page offset into an - address range or file. During system boot up, the temporary - interleaved system default policy works in this mode. - -NUMA 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, any time 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 3,5-7. If the cpuset's mems - then change to 0,2-3,5, then the interleave occurs over nodes - 0,2-3,5. - - 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, its 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 especially - 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: - -Memory Policy APIs -================== - -Linux supports 3 system calls for controlling memory policy. These APIS -always affect only the calling task, the calling task's address space, or -some shared object mapped into the calling task's address space. - -.. note:: - the headers that define these APIs and the parameter data types for - user space applications reside in a package that is not part of the - Linux kernel. The kernel system call interfaces, with the 'sys\_' - prefix, are defined in <linux/syscalls.h>; the mode and flag - definitions are defined in <linux/mempolicy.h>. - -Set [Task] Memory Policy:: - - long set_mempolicy(int mode, const unsigned long *nmask, - unsigned long maxnode); - -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. 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 - - -Get [Task] Memory Policy or Related Information:: - - long get_mempolicy(int *mode, - const unsigned long *nmask, unsigned long maxnode, - void *addr, int flags); - -Queries the "task/process memory policy" of the calling task, or the -policy or location of a specified virtual address, depending on the -'flags' argument. - -See the get_mempolicy(2) man page for more details - - -Install VMA/Shared Policy for a Range of Task's Address Space:: - - long mbind(void *start, unsigned long len, int mode, - const unsigned long *nmask, unsigned long maxnode, - unsigned flags); - -mbind() installs the policy specified by (mode, nmask, maxnodes) as a -VMA policy for the range of the calling task's address space specified -by the 'start' and 'len' arguments. Additional actions may be -requested via the 'flags' argument. - -See the mbind(2) man page for more details. - -Memory Policy Command Line Interface -==================================== - -Although not strictly part of the Linux implementation of memory policy, -a command line tool, numactl(8), exists that allows one to: - -+ set the task policy for a specified program via set_mempolicy(2), fork(2) and - exec(2) - -+ set the shared policy for a shared memory segment via mbind(2) - -The numactl(8) tool is packaged with the run-time version of the library -containing the memory policy system call wrappers. Some distributions -package the headers and compile-time libraries in a separate development -package. - -.. _mem_pol_and_cpusets: - -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 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. |