sched: add CFS documentation

add Documentation/sched-design-CFS.txt
Signed-off-by: Ingo Molnar <mingo@elte.hu>

Documentation/sched-design-CFS.txt

+
+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:
+
+         /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.
+