Date:	Wed, 20 Aug 2008 15:24:12 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	claudio@...dence.eu.com
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu
Subject: Re: [PATCH 1/1] New documentation about CFS.

On Wed, 2008-08-20 at 15:18 +0200, claudio@...dence.eu.com wrote:
> From: Claudio Scordino <claudio@...dence.eu.com>

A little changelog doesn't hurt..

Rewrite of the CFS documentation - because the old one was sorely
out-dated.

> Signed-off-by: Claudio Scordino <claudio@...dence.eu.com>
Acked-by: Peter Zijlstra <a.p.zijlstra@...llo.nl>

Thanks!

> ---
>  Documentation/scheduler/sched-design-CFS.txt |  371 +++++++++++++++-----------
>  1 files changed, 218 insertions(+), 153 deletions(-)
> 
> diff --git a/Documentation/scheduler/sched-design-CFS.txt b/Documentation/scheduler/sched-design-CFS.txt
> index 88bcb87..df601f4 100644
> --- a/Documentation/scheduler/sched-design-CFS.txt
> +++ b/Documentation/scheduler/sched-design-CFS.txt
> @@ -1,151 +1,218 @@
> +                      =============
> +                      CFS Scheduler
> +                      =============
> +
> +
> +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.
> +
> +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.
> +
> +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.
> +
> +
> +
> +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" (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.
> +
> +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.
>  
> -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
> -   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!
> -
> -   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. ]
> -
> -   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):
> -
> -         /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.
>  
> -Only one of these options to group tasks can be chosen and not both.
>  
> -Group scheduler tunables:
> +5.  SCHEDULING CLASSES
>  
> -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.
> +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.
> +
> +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.
> +
> +
> +
> +6.  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
> @@ -155,16 +222,14 @@ each new user and a "cpu_share" file is added in that directory.
>  	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

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