|
@@ -0,0 +1,119 @@
|
|
|
+
|
|
|
+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.
|
|
|
+
|