sched: add CFS documentation

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.