Merge branch 'sched-v28-for-linus' of...

mkdir $ARGV[0],0777;

$state = 0;

while (<STDIN>) {

    if (/^\.TH \"[^\"]*\" 4 \"([^\"]*)\"/) {

    if (/^\.TH \"[^\"]*\" 9 \"([^\"]*)\"/) {

	if ($state == 1) { close OUT }

	$state = 1;

	$fn = "$ARGV[0]/$1.4";

	$fn = "$ARGV[0]/$1.9";

	print STDERR "Creating $fn\n";

	open OUT, ">$fn" or die "can't open $fn: $!\n";

	print OUT $_;

                      =============

                      CFS Scheduler

                      =============

This is the CFS scheduler.

80% of CFS's design can be summed up in a single sentence: CFS basically

models an "ideal, precise multi-tasking CPU" on real hardware.

"Ideal multi-tasking CPU" is a (non-existent  :-))  CPU that has 100%

physical power and which can run each task at precise equal speed, in

parallel, each at 1/nr_running speed. For example: if there are 2 tasks

running then it runs each at 50% physical power - totally in parallel.

On real hardware, we can run only a single task at once, so while that

one task runs, the other tasks that are waiting for the CPU are at a

disadvantage - the current task gets an unfair amount of CPU time. In

CFS this fairness imbalance is expressed and tracked via the per-task

p->wait_runtime (nanosec-unit) value. "wait_runtime" is the amount of

time the task should now run on the CPU for it to become completely fair

and balanced.

( small detail: on 'ideal' hardware, the p->wait_runtime value would

  always be zero - no task would ever get 'out of balance' from the

  'ideal' share of CPU time. )

CFS's task picking logic is based on this p->wait_runtime value and it

is thus very simple: it always tries to run the task with the largest

p->wait_runtime value. In other words, CFS tries to run the task with

the 'gravest need' for more CPU time. So CFS always tries to split up

CPU time between runnable tasks as close to 'ideal multitasking

hardware' as possible.

Most of the rest of CFS's design just falls out of this really simple

concept, with a few add-on embellishments like nice levels,

multiprocessing and various algorithm variants to recognize sleepers.

In practice it works like this: the system runs a task a bit, and when

the task schedules (or a scheduler tick happens) the task's CPU usage is

'accounted for': the (small) time it just spent using the physical CPU

is deducted from p->wait_runtime. [minus the 'fair share' it would have

gotten anyway]. Once p->wait_runtime gets low enough so that another

task becomes the 'leftmost task' of the time-ordered rbtree it maintains

(plus a small amount of 'granularity' distance relative to the leftmost

task so that we do not over-schedule tasks and trash the cache) then the

new leftmost task is picked and the current task is preempted.

The rq->fair_clock value tracks the 'CPU time a runnable task would have

fairly gotten, had it been runnable during that time'. So by using

rq->fair_clock values we can accurately timestamp and measure the

'expected CPU time' a task should have gotten. All runnable tasks are

sorted in the rbtree by the "rq->fair_clock - p->wait_runtime" key, and

CFS picks the 'leftmost' task and sticks to it. As the system progresses

forwards, newly woken tasks are put into the tree more and more to the

right - slowly but surely giving a chance for every task to become the

'leftmost task' and thus get on the CPU within a deterministic amount of

time.

Some implementation details:

 - the introduction of Scheduling Classes: an extensible hierarchy of

   scheduler modules. These modules encapsulate scheduling policy

   details and are handled by the scheduler core without the core

   code assuming about them too much.

 - sched_fair.c implements the 'CFS desktop scheduler': it is a

   replacement for the vanilla scheduler's SCHED_OTHER interactivity

1.  OVERVIEW

CFS stands for "Completely Fair Scheduler," and is the new "desktop" process

scheduler implemented by Ingo Molnar and merged in Linux 2.6.23.  It is the

replacement for the previous vanilla scheduler's SCHED_OTHER interactivity

code.

   I'd like to give credit to Con Kolivas for the general approach here:

   he has proven via RSDL/SD that 'fair scheduling' is possible and that

   it results in better desktop scheduling. Kudos Con!

80% of CFS's design can be summed up in a single sentence: CFS basically models

an "ideal, precise multi-tasking CPU" on real hardware.

"Ideal multi-tasking CPU" is a (non-existent  :-)) CPU that has 100% physical

power and which can run each task at precise equal speed, in parallel, each at

1/nr_running speed.  For example: if there are 2 tasks running, then it runs

each at 50% physical power --- i.e., actually in parallel.

On real hardware, we can run only a single task at once, so we have to

introduce the concept of "virtual runtime."  The virtual runtime of a task

specifies when its next timeslice would start execution on the ideal

multi-tasking CPU described above.  In practice, the virtual runtime of a task

is its actual runtime normalized to the total number of running tasks.

2.  FEW IMPLEMENTATION DETAILS

In CFS the virtual runtime is expressed and tracked via the per-task

p->se.vruntime (nanosec-unit) value.  This way, it's possible to accurately

timestamp and measure the "expected CPU time" a task should have gotten.

[ small detail: on "ideal" hardware, at any time all tasks would have the same

  p->se.vruntime value --- i.e., tasks would execute simultaneously and no task

  would ever get "out of balance" from the "ideal" share of CPU time.  ]

CFS's task picking logic is based on this p->se.vruntime value and it is thus

very simple: it always tries to run the task with the smallest p->se.vruntime

value (i.e., the task which executed least so far).  CFS always tries to split

up CPU time between runnable tasks as close to "ideal multitasking hardware" as

possible.

Most of the rest of CFS's design just falls out of this really simple concept,

with a few add-on embellishments like nice levels, multiprocessing and various

algorithm variants to recognize sleepers.

3.  THE RBTREE

CFS's design is quite radical: it does not use the old data structures for the

runqueues, but it uses a time-ordered rbtree to build a "timeline" of future

task execution, and thus has no "array switch" artifacts (by which both the

previous vanilla scheduler and RSDL/SD are affected).

CFS also maintains the rq->cfs.min_vruntime value, which is a monotonic

increasing value tracking the smallest vruntime among all tasks in the

runqueue.  The total amount of work done by the system is tracked using

min_vruntime; that value is used to place newly activated entities on the left

side of the tree as much as possible.

The total number of running tasks in the runqueue is accounted through the

rq->cfs.load value, which is the sum of the weights of the tasks queued on the

runqueue.

CFS maintains a time-ordered rbtree, where all runnable tasks are sorted by the

p->se.vruntime key (there is a subtraction using rq->cfs.min_vruntime to

account for possible wraparounds).  CFS picks the "leftmost" task from this

tree and sticks to it.

As the system progresses forwards, the executed tasks are put into the tree

more and more to the right --- slowly but surely giving a chance for every task

to become the "leftmost task" and thus get on the CPU within a deterministic

amount of time.

   The CFS patch uses a completely different approach and implementation

   from RSDL/SD. My goal was to make CFS's interactivity quality exceed

   that of RSDL/SD, which is a high standard to meet :-) Testing

   feedback is welcome to decide this one way or another. [ and, in any

   case, all of SD's logic could be added via a kernel/sched_sd.c module

   as well, if Con is interested in such an approach. ]

Summing up, CFS works like this: it runs a task a bit, and when the task

schedules (or a scheduler tick happens) the task's CPU usage is "accounted

for": the (small) time it just spent using the physical CPU is added to

p->se.vruntime.  Once p->se.vruntime gets high enough so that another task

becomes the "leftmost task" of the time-ordered rbtree it maintains (plus a

small amount of "granularity" distance relative to the leftmost task so that we

do not over-schedule tasks and trash the cache), then the new leftmost task is

picked and the current task is preempted.

   CFS's design is quite radical: it does not use runqueues, it uses a

   time-ordered rbtree to build a 'timeline' of future task execution,

   and thus has no 'array switch' artifacts (by which both the vanilla

   scheduler and RSDL/SD are affected).

   CFS uses nanosecond granularity accounting and does not rely on any

   jiffies or other HZ detail. Thus the CFS scheduler has no notion of

   'timeslices' and has no heuristics whatsoever. There is only one

   central tunable (you have to switch on CONFIG_SCHED_DEBUG):

4.  SOME FEATURES OF CFS

CFS uses nanosecond granularity accounting and does not rely on any jiffies or

other HZ detail.  Thus the CFS scheduler has no notion of "timeslices" in the

way the previous scheduler had, and has no heuristics whatsoever.  There is

only one central tunable (you have to switch on CONFIG_SCHED_DEBUG):

   /proc/sys/kernel/sched_granularity_ns

   which can be used to tune the scheduler from 'desktop' (low

   latencies) to 'server' (good batching) workloads. It defaults to a

   setting suitable for desktop workloads. SCHED_BATCH is handled by the

   CFS scheduler module too.

   Due to its design, the CFS scheduler is not prone to any of the

   'attacks' that exist today against the heuristics of the stock

   scheduler: fiftyp.c, thud.c, chew.c, ring-test.c, massive_intr.c all

   work fine and do not impact interactivity and produce the expected

   behavior.

   the CFS scheduler has a much stronger handling of nice levels and

   SCHED_BATCH: both types of workloads should be isolated much more

   agressively than under the vanilla scheduler.

   ( another detail: due to nanosec accounting and timeline sorting,

     sched_yield() support is very simple under CFS, and in fact under

     CFS sched_yield() behaves much better than under any other

     scheduler i have tested so far. )

 - sched_rt.c implements SCHED_FIFO and SCHED_RR semantics, in a simpler

   way than the vanilla scheduler does. It uses 100 runqueues (for all

   100 RT priority levels, instead of 140 in the vanilla scheduler)

   and it needs no expired array.

 - reworked/sanitized SMP load-balancing: the runqueue-walking

   assumptions are gone from the load-balancing code now, and

   iterators of the scheduling modules are used. The balancing code got

   quite a bit simpler as a result.

Group scheduler extension to CFS

================================

Normally the scheduler operates on individual tasks and strives to provide

fair CPU time to each task. Sometimes, it may be desirable to group tasks

and provide fair CPU time to each such task group. For example, it may

be desirable to first provide fair CPU time to each user on the system

and then to each task belonging to a user.

CONFIG_FAIR_GROUP_SCHED strives to achieve exactly that. It lets

SCHED_NORMAL/BATCH tasks be be grouped and divides CPU time fairly among such

groups. At present, there are two (mutually exclusive) mechanisms to group

tasks for CPU bandwidth control purpose:

	- Based on user id (CONFIG_FAIR_USER_SCHED)

		In this option, tasks are grouped according to their user id.

	- Based on "cgroup" pseudo filesystem (CONFIG_FAIR_CGROUP_SCHED)

		This options lets the administrator create arbitrary groups

		of tasks, using the "cgroup" pseudo filesystem. See

		Documentation/cgroups.txt for more information about this

		filesystem.

which can be used to tune the scheduler from "desktop" (i.e., low latencies) to

"server" (i.e., good batching) workloads.  It defaults to a setting suitable

for desktop workloads.  SCHED_BATCH is handled by the CFS scheduler module too.

Only one of these options to group tasks can be chosen and not both.

Due to its design, the CFS scheduler is not prone to any of the "attacks" that

exist today against the heuristics of the stock scheduler: fiftyp.c, thud.c,

chew.c, ring-test.c, massive_intr.c all work fine and do not impact

interactivity and produce the expected behavior.

The CFS scheduler has a much stronger handling of nice levels and SCHED_BATCH

than the previous vanilla scheduler: both types of workloads are isolated much

more aggressively.

SMP load-balancing has been reworked/sanitized: the runqueue-walking

assumptions are gone from the load-balancing code now, and iterators of the

scheduling modules are used.  The balancing code got quite a bit simpler as a

result.

5. Scheduling policies

CFS implements three scheduling policies:

  - SCHED_NORMAL (traditionally called SCHED_OTHER): The scheduling

    policy that is used for regular tasks.

  - SCHED_BATCH: Does not preempt nearly as often as regular tasks

    would, thereby allowing tasks to run longer and make better use of

    caches but at the cost of interactivity. This is well suited for

    batch jobs.

  - SCHED_IDLE: This is even weaker than nice 19, but its not a true

    idle timer scheduler in order to avoid to get into priority

    inversion problems which would deadlock the machine.

SCHED_FIFO/_RR are implemented in sched_rt.c and are as specified by

POSIX.

The command chrt from util-linux-ng 2.13.1.1 can set all of these except

SCHED_IDLE.

6.  SCHEDULING CLASSES

Group scheduler tunables:

The new CFS scheduler has been designed in such a way to introduce "Scheduling

Classes," an extensible hierarchy of scheduler modules.  These modules

encapsulate scheduling policy details and are handled by the scheduler core

without the core code assuming too much about them.

When CONFIG_FAIR_USER_SCHED is defined, a directory is created in sysfs for

each new user and a "cpu_share" file is added in that directory.

sched_fair.c implements the CFS scheduler described above.

sched_rt.c implements SCHED_FIFO and SCHED_RR semantics, in a simpler way than

the previous vanilla scheduler did.  It uses 100 runqueues (for all 100 RT

priority levels, instead of 140 in the previous scheduler) and it needs no

expired array.

Scheduling classes are implemented through the sched_class structure, which

contains hooks to functions that must be called whenever an interesting event

occurs.

This is the (partial) list of the hooks:

 - enqueue_task(...)

   Called when a task enters a runnable state.

   It puts the scheduling entity (task) into the red-black tree and

   increments the nr_running variable.

 - dequeue_tree(...)

   When a task is no longer runnable, this function is called to keep the

   corresponding scheduling entity out of the red-black tree.  It decrements

   the nr_running variable.

 - yield_task(...)

   This function is basically just a dequeue followed by an enqueue, unless the

   compat_yield sysctl is turned on; in that case, it places the scheduling

   entity at the right-most end of the red-black tree.

 - check_preempt_curr(...)

   This function checks if a task that entered the runnable state should

   preempt the currently running task.

 - pick_next_task(...)

   This function chooses the most appropriate task eligible to run next.

 - set_curr_task(...)

   This function is called when a task changes its scheduling class or changes

   its task group.

 - task_tick(...)

   This function is mostly called from time tick functions; it might lead to

   process switch.  This drives the running preemption.

 - task_new(...)

   The core scheduler gives the scheduling module an opportunity to manage new

   task startup.  The CFS scheduling module uses it for group scheduling, while

   the scheduling module for a real-time task does not use it.

7.  GROUP SCHEDULER EXTENSIONS TO CFS

Normally, the scheduler operates on individual tasks and strives to provide

fair CPU time to each task.  Sometimes, it may be desirable to group tasks and

provide fair CPU time to each such task group.  For example, it may be

desirable to first provide fair CPU time to each user on the system and then to

each task belonging to a user.

CONFIG_GROUP_SCHED strives to achieve exactly that.  It lets tasks to be

grouped and divides CPU time fairly among such groups.

CONFIG_RT_GROUP_SCHED permits to group real-time (i.e., SCHED_FIFO and

SCHED_RR) tasks.

CONFIG_FAIR_GROUP_SCHED permits to group CFS (i.e., SCHED_NORMAL and

SCHED_BATCH) tasks.

At present, there are two (mutually exclusive) mechanisms to group tasks for

CPU bandwidth control purposes:

 - Based on user id (CONFIG_USER_SCHED)

   With this option, tasks are grouped according to their user id.

 - Based on "cgroup" pseudo filesystem (CONFIG_CGROUP_SCHED)

   This options needs CONFIG_CGROUPS to be defined, and lets the administrator

   create arbitrary groups of tasks, using the "cgroup" pseudo filesystem.  See

   Documentation/cgroups.txt for more information about this filesystem.

Only one of these options to group tasks can be chosen and not both.

When CONFIG_USER_SCHED is defined, a directory is created in sysfs for each new

user and a "cpu_share" file is added in that directory.

	# cd /sys/kernel/uids

	# cat 512/cpu_share		# Display user 512's CPU share

	2048

	#

CPU bandwidth between two users are divided in the ratio of their CPU shares.

For ex: if you would like user "root" to get twice the bandwidth of user

"guest", then set the cpu_share for both the users such that "root"'s

cpu_share is twice "guest"'s cpu_share

CPU bandwidth between two users is divided in the ratio of their CPU shares.

For example: if you would like user "root" to get twice the bandwidth of user

"guest," then set the cpu_share for both the users such that "root"'s cpu_share

is twice "guest"'s cpu_share.

When CONFIG_FAIR_CGROUP_SCHED is defined, a "cpu.shares" file is created

for each group created using the pseudo filesystem. See example steps

below to create task groups and modify their CPU share using the "cgroups"

pseudo filesystem

When CONFIG_CGROUP_SCHED is defined, a "cpu.shares" file is created for each

group created using the pseudo filesystem.  See example steps below to create

task groups and modify their CPU share using the "cgroups" pseudo filesystem.

	# mkdir /dev/cpuctl

	# mount -t cgroup -ocpu none /dev/cpuctl

	atomic_inc(&init_mm.mm_count);

	current->active_mm = &init_mm;

	/* inform the notifiers about the new cpu */

	notify_cpu_starting(cpuid);

	/* Must have completely accurate bogos.  */

	local_irq_enable();

	/*

	 * Enable local interrupts.

	 */

	notify_cpu_starting(cpu);

	local_irq_enable();

	local_fiq_enable();

	unmask_irq(IPI_INTR_VECT);

	unmask_irq(TIMER0_INTR_VECT);

	preempt_disable();

	notify_cpu_starting(cpu);

	local_irq_enable();

	cpu_set(cpu, cpu_online_map);