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Based on kernel version 4.1. Page generated on 2015-06-28 12:08 EST.

1					CPUSETS
2					-------
3	
4	Copyright (C) 2004 BULL SA.
5	Written by Simon.Derr@bull.net
6	
7	Portions Copyright (c) 2004-2006 Silicon Graphics, Inc.
8	Modified by Paul Jackson <pj@sgi.com>
9	Modified by Christoph Lameter <clameter@sgi.com>
10	Modified by Paul Menage <menage@google.com>
11	Modified by Hidetoshi Seto <seto.hidetoshi@jp.fujitsu.com>
12	
13	CONTENTS:
14	=========
15	
16	1. Cpusets
17	  1.1 What are cpusets ?
18	  1.2 Why are cpusets needed ?
19	  1.3 How are cpusets implemented ?
20	  1.4 What are exclusive cpusets ?
21	  1.5 What is memory_pressure ?
22	  1.6 What is memory spread ?
23	  1.7 What is sched_load_balance ?
24	  1.8 What is sched_relax_domain_level ?
25	  1.9 How do I use cpusets ?
26	2. Usage Examples and Syntax
27	  2.1 Basic Usage
28	  2.2 Adding/removing cpus
29	  2.3 Setting flags
30	  2.4 Attaching processes
31	3. Questions
32	4. Contact
33	
34	1. Cpusets
35	==========
36	
37	1.1 What are cpusets ?
38	----------------------
39	
40	Cpusets provide a mechanism for assigning a set of CPUs and Memory
41	Nodes to a set of tasks.   In this document "Memory Node" refers to
42	an on-line node that contains memory.
43	
44	Cpusets constrain the CPU and Memory placement of tasks to only
45	the resources within a task's current cpuset.  They form a nested
46	hierarchy visible in a virtual file system.  These are the essential
47	hooks, beyond what is already present, required to manage dynamic
48	job placement on large systems.
49	
50	Cpusets use the generic cgroup subsystem described in
51	Documentation/cgroups/cgroups.txt.
52	
53	Requests by a task, using the sched_setaffinity(2) system call to
54	include CPUs in its CPU affinity mask, and using the mbind(2) and
55	set_mempolicy(2) system calls to include Memory Nodes in its memory
56	policy, are both filtered through that task's cpuset, filtering out any
57	CPUs or Memory Nodes not in that cpuset.  The scheduler will not
58	schedule a task on a CPU that is not allowed in its cpus_allowed
59	vector, and the kernel page allocator will not allocate a page on a
60	node that is not allowed in the requesting task's mems_allowed vector.
61	
62	User level code may create and destroy cpusets by name in the cgroup
63	virtual file system, manage the attributes and permissions of these
64	cpusets and which CPUs and Memory Nodes are assigned to each cpuset,
65	specify and query to which cpuset a task is assigned, and list the
66	task pids assigned to a cpuset.
67	
68	
69	1.2 Why are cpusets needed ?
70	----------------------------
71	
72	The management of large computer systems, with many processors (CPUs),
73	complex memory cache hierarchies and multiple Memory Nodes having
74	non-uniform access times (NUMA) presents additional challenges for
75	the efficient scheduling and memory placement of processes.
76	
77	Frequently more modest sized systems can be operated with adequate
78	efficiency just by letting the operating system automatically share
79	the available CPU and Memory resources amongst the requesting tasks.
80	
81	But larger systems, which benefit more from careful processor and
82	memory placement to reduce memory access times and contention,
83	and which typically represent a larger investment for the customer,
84	can benefit from explicitly placing jobs on properly sized subsets of
85	the system.
86	
87	This can be especially valuable on:
88	
89	    * Web Servers running multiple instances of the same web application,
90	    * Servers running different applications (for instance, a web server
91	      and a database), or
92	    * NUMA systems running large HPC applications with demanding
93	      performance characteristics.
94	
95	These subsets, or "soft partitions" must be able to be dynamically
96	adjusted, as the job mix changes, without impacting other concurrently
97	executing jobs. The location of the running jobs pages may also be moved
98	when the memory locations are changed.
99	
100	The kernel cpuset patch provides the minimum essential kernel
101	mechanisms required to efficiently implement such subsets.  It
102	leverages existing CPU and Memory Placement facilities in the Linux
103	kernel to avoid any additional impact on the critical scheduler or
104	memory allocator code.
105	
106	
107	1.3 How are cpusets implemented ?
108	---------------------------------
109	
110	Cpusets provide a Linux kernel mechanism to constrain which CPUs and
111	Memory Nodes are used by a process or set of processes.
112	
113	The Linux kernel already has a pair of mechanisms to specify on which
114	CPUs a task may be scheduled (sched_setaffinity) and on which Memory
115	Nodes it may obtain memory (mbind, set_mempolicy).
116	
117	Cpusets extends these two mechanisms as follows:
118	
119	 - Cpusets are sets of allowed CPUs and Memory Nodes, known to the
120	   kernel.
121	 - Each task in the system is attached to a cpuset, via a pointer
122	   in the task structure to a reference counted cgroup structure.
123	 - Calls to sched_setaffinity are filtered to just those CPUs
124	   allowed in that task's cpuset.
125	 - Calls to mbind and set_mempolicy are filtered to just
126	   those Memory Nodes allowed in that task's cpuset.
127	 - The root cpuset contains all the systems CPUs and Memory
128	   Nodes.
129	 - For any cpuset, one can define child cpusets containing a subset
130	   of the parents CPU and Memory Node resources.
131	 - The hierarchy of cpusets can be mounted at /dev/cpuset, for
132	   browsing and manipulation from user space.
133	 - A cpuset may be marked exclusive, which ensures that no other
134	   cpuset (except direct ancestors and descendants) may contain
135	   any overlapping CPUs or Memory Nodes.
136	 - You can list all the tasks (by pid) attached to any cpuset.
137	
138	The implementation of cpusets requires a few, simple hooks
139	into the rest of the kernel, none in performance critical paths:
140	
141	 - in init/main.c, to initialize the root cpuset at system boot.
142	 - in fork and exit, to attach and detach a task from its cpuset.
143	 - in sched_setaffinity, to mask the requested CPUs by what's
144	   allowed in that task's cpuset.
145	 - in sched.c migrate_live_tasks(), to keep migrating tasks within
146	   the CPUs allowed by their cpuset, if possible.
147	 - in the mbind and set_mempolicy system calls, to mask the requested
148	   Memory Nodes by what's allowed in that task's cpuset.
149	 - in page_alloc.c, to restrict memory to allowed nodes.
150	 - in vmscan.c, to restrict page recovery to the current cpuset.
151	
152	You should mount the "cgroup" filesystem type in order to enable
153	browsing and modifying the cpusets presently known to the kernel.  No
154	new system calls are added for cpusets - all support for querying and
155	modifying cpusets is via this cpuset file system.
156	
157	The /proc/<pid>/status file for each task has four added lines,
158	displaying the task's cpus_allowed (on which CPUs it may be scheduled)
159	and mems_allowed (on which Memory Nodes it may obtain memory),
160	in the two formats seen in the following example:
161	
162	  Cpus_allowed:   ffffffff,ffffffff,ffffffff,ffffffff
163	  Cpus_allowed_list:      0-127
164	  Mems_allowed:   ffffffff,ffffffff
165	  Mems_allowed_list:      0-63
166	
167	Each cpuset is represented by a directory in the cgroup file system
168	containing (on top of the standard cgroup files) the following
169	files describing that cpuset:
170	
171	 - cpuset.cpus: list of CPUs in that cpuset
172	 - cpuset.mems: list of Memory Nodes in that cpuset
173	 - cpuset.memory_migrate flag: if set, move pages to cpusets nodes
174	 - cpuset.cpu_exclusive flag: is cpu placement exclusive?
175	 - cpuset.mem_exclusive flag: is memory placement exclusive?
176	 - cpuset.mem_hardwall flag:  is memory allocation hardwalled
177	 - cpuset.memory_pressure: measure of how much paging pressure in cpuset
178	 - cpuset.memory_spread_page flag: if set, spread page cache evenly on allowed nodes
179	 - cpuset.memory_spread_slab flag: if set, spread slab cache evenly on allowed nodes
180	 - cpuset.sched_load_balance flag: if set, load balance within CPUs on that cpuset
181	 - cpuset.sched_relax_domain_level: the searching range when migrating tasks
182	
183	In addition, only the root cpuset has the following file:
184	 - cpuset.memory_pressure_enabled flag: compute memory_pressure?
185	
186	New cpusets are created using the mkdir system call or shell
187	command.  The properties of a cpuset, such as its flags, allowed
188	CPUs and Memory Nodes, and attached tasks, are modified by writing
189	to the appropriate file in that cpusets directory, as listed above.
190	
191	The named hierarchical structure of nested cpusets allows partitioning
192	a large system into nested, dynamically changeable, "soft-partitions".
193	
194	The attachment of each task, automatically inherited at fork by any
195	children of that task, to a cpuset allows organizing the work load
196	on a system into related sets of tasks such that each set is constrained
197	to using the CPUs and Memory Nodes of a particular cpuset.  A task
198	may be re-attached to any other cpuset, if allowed by the permissions
199	on the necessary cpuset file system directories.
200	
201	Such management of a system "in the large" integrates smoothly with
202	the detailed placement done on individual tasks and memory regions
203	using the sched_setaffinity, mbind and set_mempolicy system calls.
204	
205	The following rules apply to each cpuset:
206	
207	 - Its CPUs and Memory Nodes must be a subset of its parents.
208	 - It can't be marked exclusive unless its parent is.
209	 - If its cpu or memory is exclusive, they may not overlap any sibling.
210	
211	These rules, and the natural hierarchy of cpusets, enable efficient
212	enforcement of the exclusive guarantee, without having to scan all
213	cpusets every time any of them change to ensure nothing overlaps a
214	exclusive cpuset.  Also, the use of a Linux virtual file system (vfs)
215	to represent the cpuset hierarchy provides for a familiar permission
216	and name space for cpusets, with a minimum of additional kernel code.
217	
218	The cpus and mems files in the root (top_cpuset) cpuset are
219	read-only.  The cpus file automatically tracks the value of
220	cpu_online_mask using a CPU hotplug notifier, and the mems file
221	automatically tracks the value of node_states[N_MEMORY]--i.e.,
222	nodes with memory--using the cpuset_track_online_nodes() hook.
223	
224	
225	1.4 What are exclusive cpusets ?
226	--------------------------------
227	
228	If a cpuset is cpu or mem exclusive, no other cpuset, other than
229	a direct ancestor or descendant, may share any of the same CPUs or
230	Memory Nodes.
231	
232	A cpuset that is cpuset.mem_exclusive *or* cpuset.mem_hardwall is "hardwalled",
233	i.e. it restricts kernel allocations for page, buffer and other data
234	commonly shared by the kernel across multiple users.  All cpusets,
235	whether hardwalled or not, restrict allocations of memory for user
236	space.  This enables configuring a system so that several independent
237	jobs can share common kernel data, such as file system pages, while
238	isolating each job's user allocation in its own cpuset.  To do this,
239	construct a large mem_exclusive cpuset to hold all the jobs, and
240	construct child, non-mem_exclusive cpusets for each individual job.
241	Only a small amount of typical kernel memory, such as requests from
242	interrupt handlers, is allowed to be taken outside even a
243	mem_exclusive cpuset.
244	
245	
246	1.5 What is memory_pressure ?
247	-----------------------------
248	The memory_pressure of a cpuset provides a simple per-cpuset metric
249	of the rate that the tasks in a cpuset are attempting to free up in
250	use memory on the nodes of the cpuset to satisfy additional memory
251	requests.
252	
253	This enables batch managers monitoring jobs running in dedicated
254	cpusets to efficiently detect what level of memory pressure that job
255	is causing.
256	
257	This is useful both on tightly managed systems running a wide mix of
258	submitted jobs, which may choose to terminate or re-prioritize jobs that
259	are trying to use more memory than allowed on the nodes assigned to them,
260	and with tightly coupled, long running, massively parallel scientific
261	computing jobs that will dramatically fail to meet required performance
262	goals if they start to use more memory than allowed to them.
263	
264	This mechanism provides a very economical way for the batch manager
265	to monitor a cpuset for signs of memory pressure.  It's up to the
266	batch manager or other user code to decide what to do about it and
267	take action.
268	
269	==> Unless this feature is enabled by writing "1" to the special file
270	    /dev/cpuset/memory_pressure_enabled, the hook in the rebalance
271	    code of __alloc_pages() for this metric reduces to simply noticing
272	    that the cpuset_memory_pressure_enabled flag is zero.  So only
273	    systems that enable this feature will compute the metric.
274	
275	Why a per-cpuset, running average:
276	
277	    Because this meter is per-cpuset, rather than per-task or mm,
278	    the system load imposed by a batch scheduler monitoring this
279	    metric is sharply reduced on large systems, because a scan of
280	    the tasklist can be avoided on each set of queries.
281	
282	    Because this meter is a running average, instead of an accumulating
283	    counter, a batch scheduler can detect memory pressure with a
284	    single read, instead of having to read and accumulate results
285	    for a period of time.
286	
287	    Because this meter is per-cpuset rather than per-task or mm,
288	    the batch scheduler can obtain the key information, memory
289	    pressure in a cpuset, with a single read, rather than having to
290	    query and accumulate results over all the (dynamically changing)
291	    set of tasks in the cpuset.
292	
293	A per-cpuset simple digital filter (requires a spinlock and 3 words
294	of data per-cpuset) is kept, and updated by any task attached to that
295	cpuset, if it enters the synchronous (direct) page reclaim code.
296	
297	A per-cpuset file provides an integer number representing the recent
298	(half-life of 10 seconds) rate of direct page reclaims caused by
299	the tasks in the cpuset, in units of reclaims attempted per second,
300	times 1000.
301	
302	
303	1.6 What is memory spread ?
304	---------------------------
305	There are two boolean flag files per cpuset that control where the
306	kernel allocates pages for the file system buffers and related in
307	kernel data structures.  They are called 'cpuset.memory_spread_page' and
308	'cpuset.memory_spread_slab'.
309	
310	If the per-cpuset boolean flag file 'cpuset.memory_spread_page' is set, then
311	the kernel will spread the file system buffers (page cache) evenly
312	over all the nodes that the faulting task is allowed to use, instead
313	of preferring to put those pages on the node where the task is running.
314	
315	If the per-cpuset boolean flag file 'cpuset.memory_spread_slab' is set,
316	then the kernel will spread some file system related slab caches,
317	such as for inodes and dentries evenly over all the nodes that the
318	faulting task is allowed to use, instead of preferring to put those
319	pages on the node where the task is running.
320	
321	The setting of these flags does not affect anonymous data segment or
322	stack segment pages of a task.
323	
324	By default, both kinds of memory spreading are off, and memory
325	pages are allocated on the node local to where the task is running,
326	except perhaps as modified by the task's NUMA mempolicy or cpuset
327	configuration, so long as sufficient free memory pages are available.
328	
329	When new cpusets are created, they inherit the memory spread settings
330	of their parent.
331	
332	Setting memory spreading causes allocations for the affected page
333	or slab caches to ignore the task's NUMA mempolicy and be spread
334	instead.    Tasks using mbind() or set_mempolicy() calls to set NUMA
335	mempolicies will not notice any change in these calls as a result of
336	their containing task's memory spread settings.  If memory spreading
337	is turned off, then the currently specified NUMA mempolicy once again
338	applies to memory page allocations.
339	
340	Both 'cpuset.memory_spread_page' and 'cpuset.memory_spread_slab' are boolean flag
341	files.  By default they contain "0", meaning that the feature is off
342	for that cpuset.  If a "1" is written to that file, then that turns
343	the named feature on.
344	
345	The implementation is simple.
346	
347	Setting the flag 'cpuset.memory_spread_page' turns on a per-process flag
348	PFA_SPREAD_PAGE for each task that is in that cpuset or subsequently
349	joins that cpuset.  The page allocation calls for the page cache
350	is modified to perform an inline check for this PFA_SPREAD_PAGE task
351	flag, and if set, a call to a new routine cpuset_mem_spread_node()
352	returns the node to prefer for the allocation.
353	
354	Similarly, setting 'cpuset.memory_spread_slab' turns on the flag
355	PFA_SPREAD_SLAB, and appropriately marked slab caches will allocate
356	pages from the node returned by cpuset_mem_spread_node().
357	
358	The cpuset_mem_spread_node() routine is also simple.  It uses the
359	value of a per-task rotor cpuset_mem_spread_rotor to select the next
360	node in the current task's mems_allowed to prefer for the allocation.
361	
362	This memory placement policy is also known (in other contexts) as
363	round-robin or interleave.
364	
365	This policy can provide substantial improvements for jobs that need
366	to place thread local data on the corresponding node, but that need
367	to access large file system data sets that need to be spread across
368	the several nodes in the jobs cpuset in order to fit.  Without this
369	policy, especially for jobs that might have one thread reading in the
370	data set, the memory allocation across the nodes in the jobs cpuset
371	can become very uneven.
372	
373	1.7 What is sched_load_balance ?
374	--------------------------------
375	
376	The kernel scheduler (kernel/sched/core.c) automatically load balances
377	tasks.  If one CPU is underutilized, kernel code running on that
378	CPU will look for tasks on other more overloaded CPUs and move those
379	tasks to itself, within the constraints of such placement mechanisms
380	as cpusets and sched_setaffinity.
381	
382	The algorithmic cost of load balancing and its impact on key shared
383	kernel data structures such as the task list increases more than
384	linearly with the number of CPUs being balanced.  So the scheduler
385	has support to partition the systems CPUs into a number of sched
386	domains such that it only load balances within each sched domain.
387	Each sched domain covers some subset of the CPUs in the system;
388	no two sched domains overlap; some CPUs might not be in any sched
389	domain and hence won't be load balanced.
390	
391	Put simply, it costs less to balance between two smaller sched domains
392	than one big one, but doing so means that overloads in one of the
393	two domains won't be load balanced to the other one.
394	
395	By default, there is one sched domain covering all CPUs, including those
396	marked isolated using the kernel boot time "isolcpus=" argument. However,
397	the isolated CPUs will not participate in load balancing, and will not
398	have tasks running on them unless explicitly assigned.
399	
400	This default load balancing across all CPUs is not well suited for
401	the following two situations:
402	 1) On large systems, load balancing across many CPUs is expensive.
403	    If the system is managed using cpusets to place independent jobs
404	    on separate sets of CPUs, full load balancing is unnecessary.
405	 2) Systems supporting realtime on some CPUs need to minimize
406	    system overhead on those CPUs, including avoiding task load
407	    balancing if that is not needed.
408	
409	When the per-cpuset flag "cpuset.sched_load_balance" is enabled (the default
410	setting), it requests that all the CPUs in that cpusets allowed 'cpuset.cpus'
411	be contained in a single sched domain, ensuring that load balancing
412	can move a task (not otherwised pinned, as by sched_setaffinity)
413	from any CPU in that cpuset to any other.
414	
415	When the per-cpuset flag "cpuset.sched_load_balance" is disabled, then the
416	scheduler will avoid load balancing across the CPUs in that cpuset,
417	--except-- in so far as is necessary because some overlapping cpuset
418	has "sched_load_balance" enabled.
419	
420	So, for example, if the top cpuset has the flag "cpuset.sched_load_balance"
421	enabled, then the scheduler will have one sched domain covering all
422	CPUs, and the setting of the "cpuset.sched_load_balance" flag in any other
423	cpusets won't matter, as we're already fully load balancing.
424	
425	Therefore in the above two situations, the top cpuset flag
426	"cpuset.sched_load_balance" should be disabled, and only some of the smaller,
427	child cpusets have this flag enabled.
428	
429	When doing this, you don't usually want to leave any unpinned tasks in
430	the top cpuset that might use non-trivial amounts of CPU, as such tasks
431	may be artificially constrained to some subset of CPUs, depending on
432	the particulars of this flag setting in descendant cpusets.  Even if
433	such a task could use spare CPU cycles in some other CPUs, the kernel
434	scheduler might not consider the possibility of load balancing that
435	task to that underused CPU.
436	
437	Of course, tasks pinned to a particular CPU can be left in a cpuset
438	that disables "cpuset.sched_load_balance" as those tasks aren't going anywhere
439	else anyway.
440	
441	There is an impedance mismatch here, between cpusets and sched domains.
442	Cpusets are hierarchical and nest.  Sched domains are flat; they don't
443	overlap and each CPU is in at most one sched domain.
444	
445	It is necessary for sched domains to be flat because load balancing
446	across partially overlapping sets of CPUs would risk unstable dynamics
447	that would be beyond our understanding.  So if each of two partially
448	overlapping cpusets enables the flag 'cpuset.sched_load_balance', then we
449	form a single sched domain that is a superset of both.  We won't move
450	a task to a CPU outside its cpuset, but the scheduler load balancing
451	code might waste some compute cycles considering that possibility.
452	
453	This mismatch is why there is not a simple one-to-one relation
454	between which cpusets have the flag "cpuset.sched_load_balance" enabled,
455	and the sched domain configuration.  If a cpuset enables the flag, it
456	will get balancing across all its CPUs, but if it disables the flag,
457	it will only be assured of no load balancing if no other overlapping
458	cpuset enables the flag.
459	
460	If two cpusets have partially overlapping 'cpuset.cpus' allowed, and only
461	one of them has this flag enabled, then the other may find its
462	tasks only partially load balanced, just on the overlapping CPUs.
463	This is just the general case of the top_cpuset example given a few
464	paragraphs above.  In the general case, as in the top cpuset case,
465	don't leave tasks that might use non-trivial amounts of CPU in
466	such partially load balanced cpusets, as they may be artificially
467	constrained to some subset of the CPUs allowed to them, for lack of
468	load balancing to the other CPUs.
469	
470	CPUs in "cpuset.isolcpus" were excluded from load balancing by the
471	isolcpus= kernel boot option, and will never be load balanced regardless
472	of the value of "cpuset.sched_load_balance" in any cpuset.
473	
474	1.7.1 sched_load_balance implementation details.
475	------------------------------------------------
476	
477	The per-cpuset flag 'cpuset.sched_load_balance' defaults to enabled (contrary
478	to most cpuset flags.)  When enabled for a cpuset, the kernel will
479	ensure that it can load balance across all the CPUs in that cpuset
480	(makes sure that all the CPUs in the cpus_allowed of that cpuset are
481	in the same sched domain.)
482	
483	If two overlapping cpusets both have 'cpuset.sched_load_balance' enabled,
484	then they will be (must be) both in the same sched domain.
485	
486	If, as is the default, the top cpuset has 'cpuset.sched_load_balance' enabled,
487	then by the above that means there is a single sched domain covering
488	the whole system, regardless of any other cpuset settings.
489	
490	The kernel commits to user space that it will avoid load balancing
491	where it can.  It will pick as fine a granularity partition of sched
492	domains as it can while still providing load balancing for any set
493	of CPUs allowed to a cpuset having 'cpuset.sched_load_balance' enabled.
494	
495	The internal kernel cpuset to scheduler interface passes from the
496	cpuset code to the scheduler code a partition of the load balanced
497	CPUs in the system. This partition is a set of subsets (represented
498	as an array of struct cpumask) of CPUs, pairwise disjoint, that cover
499	all the CPUs that must be load balanced.
500	
501	The cpuset code builds a new such partition and passes it to the
502	scheduler sched domain setup code, to have the sched domains rebuilt
503	as necessary, whenever:
504	 - the 'cpuset.sched_load_balance' flag of a cpuset with non-empty CPUs changes,
505	 - or CPUs come or go from a cpuset with this flag enabled,
506	 - or 'cpuset.sched_relax_domain_level' value of a cpuset with non-empty CPUs
507	   and with this flag enabled changes,
508	 - or a cpuset with non-empty CPUs and with this flag enabled is removed,
509	 - or a cpu is offlined/onlined.
510	
511	This partition exactly defines what sched domains the scheduler should
512	setup - one sched domain for each element (struct cpumask) in the
513	partition.
514	
515	The scheduler remembers the currently active sched domain partitions.
516	When the scheduler routine partition_sched_domains() is invoked from
517	the cpuset code to update these sched domains, it compares the new
518	partition requested with the current, and updates its sched domains,
519	removing the old and adding the new, for each change.
520	
521	
522	1.8 What is sched_relax_domain_level ?
523	--------------------------------------
524	
525	In sched domain, the scheduler migrates tasks in 2 ways; periodic load
526	balance on tick, and at time of some schedule events.
527	
528	When a task is woken up, scheduler try to move the task on idle CPU.
529	For example, if a task A running on CPU X activates another task B
530	on the same CPU X, and if CPU Y is X's sibling and performing idle,
531	then scheduler migrate task B to CPU Y so that task B can start on
532	CPU Y without waiting task A on CPU X.
533	
534	And if a CPU run out of tasks in its runqueue, the CPU try to pull
535	extra tasks from other busy CPUs to help them before it is going to
536	be idle.
537	
538	Of course it takes some searching cost to find movable tasks and/or
539	idle CPUs, the scheduler might not search all CPUs in the domain
540	every time.  In fact, in some architectures, the searching ranges on
541	events are limited in the same socket or node where the CPU locates,
542	while the load balance on tick searches all.
543	
544	For example, assume CPU Z is relatively far from CPU X.  Even if CPU Z
545	is idle while CPU X and the siblings are busy, scheduler can't migrate
546	woken task B from X to Z since it is out of its searching range.
547	As the result, task B on CPU X need to wait task A or wait load balance
548	on the next tick.  For some applications in special situation, waiting
549	1 tick may be too long.
550	
551	The 'cpuset.sched_relax_domain_level' file allows you to request changing
552	this searching range as you like.  This file takes int value which
553	indicates size of searching range in levels ideally as follows,
554	otherwise initial value -1 that indicates the cpuset has no request.
555	
556	  -1  : no request. use system default or follow request of others.
557	   0  : no search.
558	   1  : search siblings (hyperthreads in a core).
559	   2  : search cores in a package.
560	   3  : search cpus in a node [= system wide on non-NUMA system]
561	   4  : search nodes in a chunk of node [on NUMA system]
562	   5  : search system wide [on NUMA system]
563	
564	The system default is architecture dependent.  The system default
565	can be changed using the relax_domain_level= boot parameter.
566	
567	This file is per-cpuset and affect the sched domain where the cpuset
568	belongs to.  Therefore if the flag 'cpuset.sched_load_balance' of a cpuset
569	is disabled, then 'cpuset.sched_relax_domain_level' have no effect since
570	there is no sched domain belonging the cpuset.
571	
572	If multiple cpusets are overlapping and hence they form a single sched
573	domain, the largest value among those is used.  Be careful, if one
574	requests 0 and others are -1 then 0 is used.
575	
576	Note that modifying this file will have both good and bad effects,
577	and whether it is acceptable or not depends on your situation.
578	Don't modify this file if you are not sure.
579	
580	If your situation is:
581	 - The migration costs between each cpu can be assumed considerably
582	   small(for you) due to your special application's behavior or
583	   special hardware support for CPU cache etc.
584	 - The searching cost doesn't have impact(for you) or you can make
585	   the searching cost enough small by managing cpuset to compact etc.
586	 - The latency is required even it sacrifices cache hit rate etc.
587	then increasing 'sched_relax_domain_level' would benefit you.
588	
589	
590	1.9 How do I use cpusets ?
591	--------------------------
592	
593	In order to minimize the impact of cpusets on critical kernel
594	code, such as the scheduler, and due to the fact that the kernel
595	does not support one task updating the memory placement of another
596	task directly, the impact on a task of changing its cpuset CPU
597	or Memory Node placement, or of changing to which cpuset a task
598	is attached, is subtle.
599	
600	If a cpuset has its Memory Nodes modified, then for each task attached
601	to that cpuset, the next time that the kernel attempts to allocate
602	a page of memory for that task, the kernel will notice the change
603	in the task's cpuset, and update its per-task memory placement to
604	remain within the new cpusets memory placement.  If the task was using
605	mempolicy MPOL_BIND, and the nodes to which it was bound overlap with
606	its new cpuset, then the task will continue to use whatever subset
607	of MPOL_BIND nodes are still allowed in the new cpuset.  If the task
608	was using MPOL_BIND and now none of its MPOL_BIND nodes are allowed
609	in the new cpuset, then the task will be essentially treated as if it
610	was MPOL_BIND bound to the new cpuset (even though its NUMA placement,
611	as queried by get_mempolicy(), doesn't change).  If a task is moved
612	from one cpuset to another, then the kernel will adjust the task's
613	memory placement, as above, the next time that the kernel attempts
614	to allocate a page of memory for that task.
615	
616	If a cpuset has its 'cpuset.cpus' modified, then each task in that cpuset
617	will have its allowed CPU placement changed immediately.  Similarly,
618	if a task's pid is written to another cpusets 'cpuset.tasks' file, then its
619	allowed CPU placement is changed immediately.  If such a task had been
620	bound to some subset of its cpuset using the sched_setaffinity() call,
621	the task will be allowed to run on any CPU allowed in its new cpuset,
622	negating the effect of the prior sched_setaffinity() call.
623	
624	In summary, the memory placement of a task whose cpuset is changed is
625	updated by the kernel, on the next allocation of a page for that task,
626	and the processor placement is updated immediately.
627	
628	Normally, once a page is allocated (given a physical page
629	of main memory) then that page stays on whatever node it
630	was allocated, so long as it remains allocated, even if the
631	cpusets memory placement policy 'cpuset.mems' subsequently changes.
632	If the cpuset flag file 'cpuset.memory_migrate' is set true, then when
633	tasks are attached to that cpuset, any pages that task had
634	allocated to it on nodes in its previous cpuset are migrated
635	to the task's new cpuset. The relative placement of the page within
636	the cpuset is preserved during these migration operations if possible.
637	For example if the page was on the second valid node of the prior cpuset
638	then the page will be placed on the second valid node of the new cpuset.
639	
640	Also if 'cpuset.memory_migrate' is set true, then if that cpuset's
641	'cpuset.mems' file is modified, pages allocated to tasks in that
642	cpuset, that were on nodes in the previous setting of 'cpuset.mems',
643	will be moved to nodes in the new setting of 'mems.'
644	Pages that were not in the task's prior cpuset, or in the cpuset's
645	prior 'cpuset.mems' setting, will not be moved.
646	
647	There is an exception to the above.  If hotplug functionality is used
648	to remove all the CPUs that are currently assigned to a cpuset,
649	then all the tasks in that cpuset will be moved to the nearest ancestor
650	with non-empty cpus.  But the moving of some (or all) tasks might fail if
651	cpuset is bound with another cgroup subsystem which has some restrictions
652	on task attaching.  In this failing case, those tasks will stay
653	in the original cpuset, and the kernel will automatically update
654	their cpus_allowed to allow all online CPUs.  When memory hotplug
655	functionality for removing Memory Nodes is available, a similar exception
656	is expected to apply there as well.  In general, the kernel prefers to
657	violate cpuset placement, over starving a task that has had all
658	its allowed CPUs or Memory Nodes taken offline.
659	
660	There is a second exception to the above.  GFP_ATOMIC requests are
661	kernel internal allocations that must be satisfied, immediately.
662	The kernel may drop some request, in rare cases even panic, if a
663	GFP_ATOMIC alloc fails.  If the request cannot be satisfied within
664	the current task's cpuset, then we relax the cpuset, and look for
665	memory anywhere we can find it.  It's better to violate the cpuset
666	than stress the kernel.
667	
668	To start a new job that is to be contained within a cpuset, the steps are:
669	
670	 1) mkdir /sys/fs/cgroup/cpuset
671	 2) mount -t cgroup -ocpuset cpuset /sys/fs/cgroup/cpuset
672	 3) Create the new cpuset by doing mkdir's and write's (or echo's) in
673	    the /sys/fs/cgroup/cpuset virtual file system.
674	 4) Start a task that will be the "founding father" of the new job.
675	 5) Attach that task to the new cpuset by writing its pid to the
676	    /sys/fs/cgroup/cpuset tasks file for that cpuset.
677	 6) fork, exec or clone the job tasks from this founding father task.
678	
679	For example, the following sequence of commands will setup a cpuset
680	named "Charlie", containing just CPUs 2 and 3, and Memory Node 1,
681	and then start a subshell 'sh' in that cpuset:
682	
683	  mount -t cgroup -ocpuset cpuset /sys/fs/cgroup/cpuset
684	  cd /sys/fs/cgroup/cpuset
685	  mkdir Charlie
686	  cd Charlie
687	  /bin/echo 2-3 > cpuset.cpus
688	  /bin/echo 1 > cpuset.mems
689	  /bin/echo $$ > tasks
690	  sh
691	  # The subshell 'sh' is now running in cpuset Charlie
692	  # The next line should display '/Charlie'
693	  cat /proc/self/cpuset
694	
695	There are ways to query or modify cpusets:
696	 - via the cpuset file system directly, using the various cd, mkdir, echo,
697	   cat, rmdir commands from the shell, or their equivalent from C.
698	 - via the C library libcpuset.
699	 - via the C library libcgroup.
700	   (http://sourceforge.net/projects/libcg/)
701	 - via the python application cset.
702	   (http://code.google.com/p/cpuset/)
703	
704	The sched_setaffinity calls can also be done at the shell prompt using
705	SGI's runon or Robert Love's taskset.  The mbind and set_mempolicy
706	calls can be done at the shell prompt using the numactl command
707	(part of Andi Kleen's numa package).
708	
709	2. Usage Examples and Syntax
710	============================
711	
712	2.1 Basic Usage
713	---------------
714	
715	Creating, modifying, using the cpusets can be done through the cpuset
716	virtual filesystem.
717	
718	To mount it, type:
719	# mount -t cgroup -o cpuset cpuset /sys/fs/cgroup/cpuset
720	
721	Then under /sys/fs/cgroup/cpuset you can find a tree that corresponds to the
722	tree of the cpusets in the system. For instance, /sys/fs/cgroup/cpuset
723	is the cpuset that holds the whole system.
724	
725	If you want to create a new cpuset under /sys/fs/cgroup/cpuset:
726	# cd /sys/fs/cgroup/cpuset
727	# mkdir my_cpuset
728	
729	Now you want to do something with this cpuset.
730	# cd my_cpuset
731	
732	In this directory you can find several files:
733	# ls
734	cgroup.clone_children  cpuset.memory_pressure
735	cgroup.event_control   cpuset.memory_spread_page
736	cgroup.procs           cpuset.memory_spread_slab
737	cpuset.cpu_exclusive   cpuset.mems
738	cpuset.cpus            cpuset.sched_load_balance
739	cpuset.mem_exclusive   cpuset.sched_relax_domain_level
740	cpuset.mem_hardwall    notify_on_release
741	cpuset.memory_migrate  tasks
742	
743	Reading them will give you information about the state of this cpuset:
744	the CPUs and Memory Nodes it can use, the processes that are using
745	it, its properties.  By writing to these files you can manipulate
746	the cpuset.
747	
748	Set some flags:
749	# /bin/echo 1 > cpuset.cpu_exclusive
750	
751	Add some cpus:
752	# /bin/echo 0-7 > cpuset.cpus
753	
754	Add some mems:
755	# /bin/echo 0-7 > cpuset.mems
756	
757	Now attach your shell to this cpuset:
758	# /bin/echo $$ > tasks
759	
760	You can also create cpusets inside your cpuset by using mkdir in this
761	directory.
762	# mkdir my_sub_cs
763	
764	To remove a cpuset, just use rmdir:
765	# rmdir my_sub_cs
766	This will fail if the cpuset is in use (has cpusets inside, or has
767	processes attached).
768	
769	Note that for legacy reasons, the "cpuset" filesystem exists as a
770	wrapper around the cgroup filesystem.
771	
772	The command
773	
774	mount -t cpuset X /sys/fs/cgroup/cpuset
775	
776	is equivalent to
777	
778	mount -t cgroup -ocpuset,noprefix X /sys/fs/cgroup/cpuset
779	echo "/sbin/cpuset_release_agent" > /sys/fs/cgroup/cpuset/release_agent
780	
781	2.2 Adding/removing cpus
782	------------------------
783	
784	This is the syntax to use when writing in the cpus or mems files
785	in cpuset directories:
786	
787	# /bin/echo 1-4 > cpuset.cpus		-> set cpus list to cpus 1,2,3,4
788	# /bin/echo 1,2,3,4 > cpuset.cpus	-> set cpus list to cpus 1,2,3,4
789	
790	To add a CPU to a cpuset, write the new list of CPUs including the
791	CPU to be added. To add 6 to the above cpuset:
792	
793	# /bin/echo 1-4,6 > cpuset.cpus	-> set cpus list to cpus 1,2,3,4,6
794	
795	Similarly to remove a CPU from a cpuset, write the new list of CPUs
796	without the CPU to be removed.
797	
798	To remove all the CPUs:
799	
800	# /bin/echo "" > cpuset.cpus		-> clear cpus list
801	
802	2.3 Setting flags
803	-----------------
804	
805	The syntax is very simple:
806	
807	# /bin/echo 1 > cpuset.cpu_exclusive 	-> set flag 'cpuset.cpu_exclusive'
808	# /bin/echo 0 > cpuset.cpu_exclusive 	-> unset flag 'cpuset.cpu_exclusive'
809	
810	2.4 Attaching processes
811	-----------------------
812	
813	# /bin/echo PID > tasks
814	
815	Note that it is PID, not PIDs. You can only attach ONE task at a time.
816	If you have several tasks to attach, you have to do it one after another:
817	
818	# /bin/echo PID1 > tasks
819	# /bin/echo PID2 > tasks
820		...
821	# /bin/echo PIDn > tasks
822	
823	
824	3. Questions
825	============
826	
827	Q: what's up with this '/bin/echo' ?
828	A: bash's builtin 'echo' command does not check calls to write() against
829	   errors. If you use it in the cpuset file system, you won't be
830	   able to tell whether a command succeeded or failed.
831	
832	Q: When I attach processes, only the first of the line gets really attached !
833	A: We can only return one error code per call to write(). So you should also
834	   put only ONE pid.
835	
836	4. Contact
837	==========
838	
839	Web: http://www.bullopensource.org/cpuset
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