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Commit 8bf46a39 authored by Srivatsa Vaddagiri's avatar Srivatsa Vaddagiri Committed by Ingo Molnar
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sched/fair: Fix SCHED_HRTICK bug leading to late preemption of tasks 



SCHED_HRTICK feature is useful to preempt SCHED_FAIR tasks on-the-dot
(just when they would have exceeded their ideal_runtime).

It makes use of a per-CPU hrtimer resource and hence arming that
hrtimer should be based on total SCHED_FAIR tasks a CPU has across its
various cfs_rqs, rather than being based on number of tasks in a
particular cfs_rq (as implemented currently).

As a result, with current code, its possible for a running task (which
is the sole task in its cfs_rq) to be preempted much after its
ideal_runtime has elapsed, resulting in increased latency for tasks in
other cfs_rq on same CPU.

Fix this by arming sched hrtimer based on total number of SCHED_FAIR
tasks a CPU has across its various cfs_rqs.

Signed-off-by: default avatarSrivatsa Vaddagiri <vatsa@codeaurora.org>
Signed-off-by: default avatarJoonwoo Park <joonwoop@codeaurora.org>
Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/1474075731-11550-1-git-send-email-joonwoop@codeaurora.org


Signed-off-by: default avatarIngo Molnar <mingo@kernel.org>
parent 35a773a0

kernel/sched/fair.c

+1 −1
Original line number Diff line number Diff line
@@ -4469,7 +4469,7 @@ static void hrtick_start_fair(struct rq *rq, struct task_struct *p) 


	WARN_ON(task_rq(p) != rq);



	if (cfs_rq->nr_running > 1) {

	if (rq->cfs.h_nr_running > 1) {

		u64 slice = sched_slice(cfs_rq, se);

		u64 ran = se->sum_exec_runtime - se->prev_sum_exec_runtime;

		s64 delta = slice - ran;