![Paul Jackson](/assets/img/avatar_default.png)
This patch makes use of the previously underutilized cpuset flag 'mem_exclusive' to provide what amounts to another layer of memory placement resolution. With this patch, there are now the following four layers of memory placement available: 1) The whole system (interrupt and GFP_ATOMIC allocations can use this), 2) The nearest enclosing mem_exclusive cpuset (GFP_KERNEL allocations can use), 3) The current tasks cpuset (GFP_USER allocations constrained to here), and 4) Specific node placement, using mbind and set_mempolicy. These nest - each layer is a subset (same or within) of the previous. Layer (2) above is new, with this patch. The call used to check whether a zone (its node, actually) is in a cpuset (in its mems_allowed, actually) is extended to take a gfp_mask argument, and its logic is extended, in the case that __GFP_HARDWALL is not set in the flag bits, to look up the cpuset hierarchy for the nearest enclosing mem_exclusive cpuset, to determine if placement is allowed. The definition of GFP_USER, which used to be identical to GFP_KERNEL, is changed to also set the __GFP_HARDWALL bit, in the previous cpuset_gfp_hardwall_flag patch. GFP_ATOMIC and GFP_KERNEL allocations will stay within the current tasks cpuset, so long as any node therein is not too tight on memory, but will escape to the larger layer, if need be. The intended use is to allow something like a batch manager to handle several jobs, each job in its own cpuset, but using common kernel memory for caches and such. Swapper and oom_kill activity is also constrained to Layer (2). A task in or below one mem_exclusive cpuset should not cause swapping on nodes in another non-overlapping mem_exclusive cpuset, nor provoke oom_killing of a task in another such cpuset. Heavy use of kernel memory for i/o caching and such by one job should not impact the memory available to jobs in other non-overlapping mem_exclusive cpusets. This patch enables providing hardwall, inescapable cpusets for memory allocations of each job, while sharing kernel memory allocations between several jobs, in an enclosing mem_exclusive cpuset. Like Dinakar's patch earlier to enable administering sched domains using the cpu_exclusive flag, this patch also provides a useful meaning to a cpuset flag that had previously done nothing much useful other than restrict what cpuset configurations were allowed. Signed-off-by: Paul Jackson <pj@sgi.com> Signed-off-by: Andrew Morton <akpm@osdl.org> Signed-off-by: Linus Torvalds <torvalds@osdl.org>
443 lines
18 KiB
Plaintext
443 lines
18 KiB
Plaintext
CPUSETS
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-------
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Copyright (C) 2004 BULL SA.
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Written by Simon.Derr@bull.net
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Portions Copyright (c) 2004 Silicon Graphics, Inc.
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Modified by Paul Jackson <pj@sgi.com>
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CONTENTS:
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=========
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1. Cpusets
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1.1 What are cpusets ?
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1.2 Why are cpusets needed ?
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1.3 How are cpusets implemented ?
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1.4 How do I use cpusets ?
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2. Usage Examples and Syntax
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2.1 Basic Usage
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2.2 Adding/removing cpus
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2.3 Setting flags
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2.4 Attaching processes
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3. Questions
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4. Contact
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1. Cpusets
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==========
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1.1 What are cpusets ?
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----------------------
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Cpusets provide a mechanism for assigning a set of CPUs and Memory
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Nodes to a set of tasks.
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Cpusets constrain the CPU and Memory placement of tasks to only
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the resources within a tasks current cpuset. They form a nested
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hierarchy visible in a virtual file system. These are the essential
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hooks, beyond what is already present, required to manage dynamic
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job placement on large systems.
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Each task has a pointer to a cpuset. Multiple tasks may reference
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the same cpuset. Requests by a task, using the sched_setaffinity(2)
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system call to include CPUs in its CPU affinity mask, and using the
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mbind(2) and set_mempolicy(2) system calls to include Memory Nodes
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in its memory policy, are both filtered through that tasks cpuset,
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filtering out any CPUs or Memory Nodes not in that cpuset. The
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scheduler will not schedule a task on a CPU that is not allowed in
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its cpus_allowed vector, and the kernel page allocator will not
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allocate a page on a node that is not allowed in the requesting tasks
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mems_allowed vector.
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If a cpuset is cpu or mem exclusive, no other cpuset, other than a direct
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ancestor or descendent, may share any of the same CPUs or Memory Nodes.
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A cpuset that is cpu exclusive has a sched domain associated with it.
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The sched domain consists of all cpus in the current cpuset that are not
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part of any exclusive child cpusets.
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This ensures that the scheduler load balacing code only balances
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against the cpus that are in the sched domain as defined above and not
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all of the cpus in the system. This removes any overhead due to
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load balancing code trying to pull tasks outside of the cpu exclusive
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cpuset only to be prevented by the tasks' cpus_allowed mask.
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A cpuset that is mem_exclusive restricts kernel allocations for
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page, buffer and other data commonly shared by the kernel across
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multiple users. All cpusets, whether mem_exclusive or not, restrict
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allocations of memory for user space. This enables configuring a
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system so that several independent jobs can share common kernel
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data, such as file system pages, while isolating each jobs user
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allocation in its own cpuset. To do this, construct a large
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mem_exclusive cpuset to hold all the jobs, and construct child,
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non-mem_exclusive cpusets for each individual job. Only a small
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amount of typical kernel memory, such as requests from interrupt
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handlers, is allowed to be taken outside even a mem_exclusive cpuset.
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User level code may create and destroy cpusets by name in the cpuset
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virtual file system, manage the attributes and permissions of these
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cpusets and which CPUs and Memory Nodes are assigned to each cpuset,
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specify and query to which cpuset a task is assigned, and list the
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task pids assigned to a cpuset.
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1.2 Why are cpusets needed ?
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----------------------------
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The management of large computer systems, with many processors (CPUs),
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complex memory cache hierarchies and multiple Memory Nodes having
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non-uniform access times (NUMA) presents additional challenges for
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the efficient scheduling and memory placement of processes.
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Frequently more modest sized systems can be operated with adequate
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efficiency just by letting the operating system automatically share
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the available CPU and Memory resources amongst the requesting tasks.
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But larger systems, which benefit more from careful processor and
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memory placement to reduce memory access times and contention,
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and which typically represent a larger investment for the customer,
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can benefit from explictly placing jobs on properly sized subsets of
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the system.
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This can be especially valuable on:
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* Web Servers running multiple instances of the same web application,
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* Servers running different applications (for instance, a web server
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and a database), or
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* NUMA systems running large HPC applications with demanding
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performance characteristics.
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* Also cpu_exclusive cpusets are useful for servers running orthogonal
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workloads such as RT applications requiring low latency and HPC
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applications that are throughput sensitive
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These subsets, or "soft partitions" must be able to be dynamically
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adjusted, as the job mix changes, without impacting other concurrently
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executing jobs.
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The kernel cpuset patch provides the minimum essential kernel
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mechanisms required to efficiently implement such subsets. It
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leverages existing CPU and Memory Placement facilities in the Linux
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kernel to avoid any additional impact on the critical scheduler or
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memory allocator code.
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1.3 How are cpusets implemented ?
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---------------------------------
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Cpusets provide a Linux kernel (2.6.7 and above) mechanism to constrain
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which CPUs and Memory Nodes are used by a process or set of processes.
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The Linux kernel already has a pair of mechanisms to specify on which
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CPUs a task may be scheduled (sched_setaffinity) and on which Memory
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Nodes it may obtain memory (mbind, set_mempolicy).
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Cpusets extends these two mechanisms as follows:
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- Cpusets are sets of allowed CPUs and Memory Nodes, known to the
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kernel.
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- Each task in the system is attached to a cpuset, via a pointer
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in the task structure to a reference counted cpuset structure.
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- Calls to sched_setaffinity are filtered to just those CPUs
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allowed in that tasks cpuset.
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- Calls to mbind and set_mempolicy are filtered to just
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those Memory Nodes allowed in that tasks cpuset.
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- The root cpuset contains all the systems CPUs and Memory
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Nodes.
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- For any cpuset, one can define child cpusets containing a subset
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of the parents CPU and Memory Node resources.
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- The hierarchy of cpusets can be mounted at /dev/cpuset, for
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browsing and manipulation from user space.
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- A cpuset may be marked exclusive, which ensures that no other
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cpuset (except direct ancestors and descendents) may contain
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any overlapping CPUs or Memory Nodes.
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Also a cpu_exclusive cpuset would be associated with a sched
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domain.
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- You can list all the tasks (by pid) attached to any cpuset.
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The implementation of cpusets requires a few, simple hooks
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into the rest of the kernel, none in performance critical paths:
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- in main/init.c, to initialize the root cpuset at system boot.
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- in fork and exit, to attach and detach a task from its cpuset.
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- in sched_setaffinity, to mask the requested CPUs by what's
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allowed in that tasks cpuset.
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- in sched.c migrate_all_tasks(), to keep migrating tasks within
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the CPUs allowed by their cpuset, if possible.
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- in sched.c, a new API partition_sched_domains for handling
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sched domain changes associated with cpu_exclusive cpusets
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and related changes in both sched.c and arch/ia64/kernel/domain.c
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- in the mbind and set_mempolicy system calls, to mask the requested
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Memory Nodes by what's allowed in that tasks cpuset.
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- in page_alloc, to restrict memory to allowed nodes.
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- in vmscan.c, to restrict page recovery to the current cpuset.
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In addition a new file system, of type "cpuset" may be mounted,
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typically at /dev/cpuset, to enable browsing and modifying the cpusets
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presently known to the kernel. No new system calls are added for
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cpusets - all support for querying and modifying cpusets is via
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this cpuset file system.
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Each task under /proc has an added file named 'cpuset', displaying
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the cpuset name, as the path relative to the root of the cpuset file
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system.
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The /proc/<pid>/status file for each task has two added lines,
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displaying the tasks cpus_allowed (on which CPUs it may be scheduled)
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and mems_allowed (on which Memory Nodes it may obtain memory),
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in the format seen in the following example:
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Cpus_allowed: ffffffff,ffffffff,ffffffff,ffffffff
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Mems_allowed: ffffffff,ffffffff
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Each cpuset is represented by a directory in the cpuset file system
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containing the following files describing that cpuset:
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- cpus: list of CPUs in that cpuset
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- mems: list of Memory Nodes in that cpuset
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- cpu_exclusive flag: is cpu placement exclusive?
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- mem_exclusive flag: is memory placement exclusive?
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- tasks: list of tasks (by pid) attached to that cpuset
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New cpusets are created using the mkdir system call or shell
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command. The properties of a cpuset, such as its flags, allowed
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CPUs and Memory Nodes, and attached tasks, are modified by writing
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to the appropriate file in that cpusets directory, as listed above.
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The named hierarchical structure of nested cpusets allows partitioning
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a large system into nested, dynamically changeable, "soft-partitions".
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The attachment of each task, automatically inherited at fork by any
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children of that task, to a cpuset allows organizing the work load
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on a system into related sets of tasks such that each set is constrained
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to using the CPUs and Memory Nodes of a particular cpuset. A task
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may be re-attached to any other cpuset, if allowed by the permissions
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on the necessary cpuset file system directories.
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Such management of a system "in the large" integrates smoothly with
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the detailed placement done on individual tasks and memory regions
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using the sched_setaffinity, mbind and set_mempolicy system calls.
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The following rules apply to each cpuset:
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- Its CPUs and Memory Nodes must be a subset of its parents.
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- It can only be marked exclusive if its parent is.
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- If its cpu or memory is exclusive, they may not overlap any sibling.
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These rules, and the natural hierarchy of cpusets, enable efficient
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enforcement of the exclusive guarantee, without having to scan all
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cpusets every time any of them change to ensure nothing overlaps a
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exclusive cpuset. Also, the use of a Linux virtual file system (vfs)
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to represent the cpuset hierarchy provides for a familiar permission
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and name space for cpusets, with a minimum of additional kernel code.
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1.4 How do I use cpusets ?
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--------------------------
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In order to minimize the impact of cpusets on critical kernel
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code, such as the scheduler, and due to the fact that the kernel
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does not support one task updating the memory placement of another
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task directly, the impact on a task of changing its cpuset CPU
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or Memory Node placement, or of changing to which cpuset a task
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is attached, is subtle.
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If a cpuset has its Memory Nodes modified, then for each task attached
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to that cpuset, the next time that the kernel attempts to allocate
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a page of memory for that task, the kernel will notice the change
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in the tasks cpuset, and update its per-task memory placement to
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remain within the new cpusets memory placement. If the task was using
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mempolicy MPOL_BIND, and the nodes to which it was bound overlap with
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its new cpuset, then the task will continue to use whatever subset
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of MPOL_BIND nodes are still allowed in the new cpuset. If the task
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was using MPOL_BIND and now none of its MPOL_BIND nodes are allowed
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in the new cpuset, then the task will be essentially treated as if it
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was MPOL_BIND bound to the new cpuset (even though its numa placement,
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as queried by get_mempolicy(), doesn't change). If a task is moved
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from one cpuset to another, then the kernel will adjust the tasks
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memory placement, as above, the next time that the kernel attempts
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to allocate a page of memory for that task.
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If a cpuset has its CPUs modified, then each task using that
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cpuset does _not_ change its behavior automatically. In order to
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minimize the impact on the critical scheduling code in the kernel,
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tasks will continue to use their prior CPU placement until they
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are rebound to their cpuset, by rewriting their pid to the 'tasks'
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file of their cpuset. If a task had been bound to some subset of its
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cpuset using the sched_setaffinity() call, and if any of that subset
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is still allowed in its new cpuset settings, then the task will be
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restricted to the intersection of the CPUs it was allowed on before,
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and its new cpuset CPU placement. If, on the other hand, there is
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no overlap between a tasks prior placement and its new cpuset CPU
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placement, then the task will be allowed to run on any CPU allowed
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in its new cpuset. If a task is moved from one cpuset to another,
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its CPU placement is updated in the same way as if the tasks pid is
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rewritten to the 'tasks' file of its current cpuset.
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In summary, the memory placement of a task whose cpuset is changed is
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updated by the kernel, on the next allocation of a page for that task,
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but the processor placement is not updated, until that tasks pid is
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rewritten to the 'tasks' file of its cpuset. This is done to avoid
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impacting the scheduler code in the kernel with a check for changes
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in a tasks processor placement.
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There is an exception to the above. If hotplug funtionality is used
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to remove all the CPUs that are currently assigned to a cpuset,
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then the kernel will automatically update the cpus_allowed of all
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tasks attached to CPUs in that cpuset to allow all CPUs. When memory
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hotplug functionality for removing Memory Nodes is available, a
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similar exception is expected to apply there as well. In general,
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the kernel prefers to violate cpuset placement, over starving a task
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that has had all its allowed CPUs or Memory Nodes taken offline. User
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code should reconfigure cpusets to only refer to online CPUs and Memory
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Nodes when using hotplug to add or remove such resources.
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There is a second exception to the above. GFP_ATOMIC requests are
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kernel internal allocations that must be satisfied, immediately.
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The kernel may drop some request, in rare cases even panic, if a
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GFP_ATOMIC alloc fails. If the request cannot be satisfied within
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the current tasks cpuset, then we relax the cpuset, and look for
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memory anywhere we can find it. It's better to violate the cpuset
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than stress the kernel.
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To start a new job that is to be contained within a cpuset, the steps are:
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1) mkdir /dev/cpuset
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2) mount -t cpuset none /dev/cpuset
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3) Create the new cpuset by doing mkdir's and write's (or echo's) in
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the /dev/cpuset virtual file system.
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4) Start a task that will be the "founding father" of the new job.
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5) Attach that task to the new cpuset by writing its pid to the
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/dev/cpuset tasks file for that cpuset.
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6) fork, exec or clone the job tasks from this founding father task.
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For example, the following sequence of commands will setup a cpuset
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named "Charlie", containing just CPUs 2 and 3, and Memory Node 1,
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and then start a subshell 'sh' in that cpuset:
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mount -t cpuset none /dev/cpuset
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cd /dev/cpuset
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mkdir Charlie
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cd Charlie
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/bin/echo 2-3 > cpus
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/bin/echo 1 > mems
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/bin/echo $$ > tasks
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sh
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# The subshell 'sh' is now running in cpuset Charlie
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# The next line should display '/Charlie'
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cat /proc/self/cpuset
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In the case that a change of cpuset includes wanting to move already
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allocated memory pages, consider further the work of IWAMOTO
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Toshihiro <iwamoto@valinux.co.jp> for page remapping and memory
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hotremoval, which can be found at:
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http://people.valinux.co.jp/~iwamoto/mh.html
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The integration of cpusets with such memory migration is not yet
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available.
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In the future, a C library interface to cpusets will likely be
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available. For now, the only way to query or modify cpusets is
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via the cpuset file system, using the various cd, mkdir, echo, cat,
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rmdir commands from the shell, or their equivalent from C.
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The sched_setaffinity calls can also be done at the shell prompt using
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SGI's runon or Robert Love's taskset. The mbind and set_mempolicy
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calls can be done at the shell prompt using the numactl command
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(part of Andi Kleen's numa package).
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2. Usage Examples and Syntax
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============================
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2.1 Basic Usage
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---------------
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Creating, modifying, using the cpusets can be done through the cpuset
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virtual filesystem.
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To mount it, type:
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# mount -t cpuset none /dev/cpuset
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Then under /dev/cpuset you can find a tree that corresponds to the
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tree of the cpusets in the system. For instance, /dev/cpuset
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is the cpuset that holds the whole system.
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If you want to create a new cpuset under /dev/cpuset:
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# cd /dev/cpuset
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# mkdir my_cpuset
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Now you want to do something with this cpuset.
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# cd my_cpuset
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In this directory you can find several files:
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# ls
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cpus cpu_exclusive mems mem_exclusive tasks
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Reading them will give you information about the state of this cpuset:
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the CPUs and Memory Nodes it can use, the processes that are using
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it, its properties. By writing to these files you can manipulate
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the cpuset.
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Set some flags:
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# /bin/echo 1 > cpu_exclusive
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Add some cpus:
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# /bin/echo 0-7 > cpus
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Now attach your shell to this cpuset:
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# /bin/echo $$ > tasks
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You can also create cpusets inside your cpuset by using mkdir in this
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directory.
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# mkdir my_sub_cs
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To remove a cpuset, just use rmdir:
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# rmdir my_sub_cs
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This will fail if the cpuset is in use (has cpusets inside, or has
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processes attached).
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2.2 Adding/removing cpus
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------------------------
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This is the syntax to use when writing in the cpus or mems files
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in cpuset directories:
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# /bin/echo 1-4 > cpus -> set cpus list to cpus 1,2,3,4
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# /bin/echo 1,2,3,4 > cpus -> set cpus list to cpus 1,2,3,4
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2.3 Setting flags
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-----------------
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The syntax is very simple:
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# /bin/echo 1 > cpu_exclusive -> set flag 'cpu_exclusive'
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# /bin/echo 0 > cpu_exclusive -> unset flag 'cpu_exclusive'
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2.4 Attaching processes
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-----------------------
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# /bin/echo PID > tasks
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Note that it is PID, not PIDs. You can only attach ONE task at a time.
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If you have several tasks to attach, you have to do it one after another:
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# /bin/echo PID1 > tasks
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# /bin/echo PID2 > tasks
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...
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# /bin/echo PIDn > tasks
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3. Questions
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============
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Q: what's up with this '/bin/echo' ?
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A: bash's builtin 'echo' command does not check calls to write() against
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errors. If you use it in the cpuset file system, you won't be
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able to tell whether a command succeeded or failed.
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Q: When I attach processes, only the first of the line gets really attached !
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A: We can only return one error code per call to write(). So you should also
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put only ONE pid.
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4. Contact
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==========
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Web: http://www.bullopensource.org/cpuset
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