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diff --git a/Documentation/scheduler/sched-design-CFS.rst b/Documentation/scheduler/sched-design-CFS.rst new file mode 100644 index 000000000000..53b30d1967cf --- /dev/null +++ b/Documentation/scheduler/sched-design-CFS.rst @@ -0,0 +1,249 @@ +============= +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. 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_min_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. + + + +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 +====================== + +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_task(...) + + 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. + + + + +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_CGROUP_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. + + These options need CONFIG_CGROUPS to be defined, and let the administrator + create arbitrary groups of tasks, using the "cgroup" pseudo filesystem. See + Documentation/cgroup-v1/cgroups.rst for more information about this filesystem. + +When CONFIG_FAIR_GROUP_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:: + + # mount -t tmpfs cgroup_root /sys/fs/cgroup + # mkdir /sys/fs/cgroup/cpu + # mount -t cgroup -ocpu none /sys/fs/cgroup/cpu + # cd /sys/fs/cgroup/cpu + + # mkdir multimedia # create "multimedia" group of tasks + # mkdir browser # create "browser" group of tasks + + # #Configure the multimedia group to receive twice the CPU bandwidth + # #that of browser group + + # echo 2048 > multimedia/cpu.shares + # echo 1024 > browser/cpu.shares + + # firefox & # Launch firefox and move it to "browser" group + # echo <firefox_pid> > browser/tasks + + # #Launch gmplayer (or your favourite movie player) + # echo <movie_player_pid> > multimedia/tasks |