Messages in this thread Patch in this message | | | From | Joonwoo Park <> | Subject | [PATCH] sched: Fix SCHED_HRTICK bug leading to late preemption of tasks | Date | Fri, 16 Sep 2016 18:28:51 -0700 |
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From: Srivatsa Vaddagiri <vatsa@codeaurora.org>
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 alarming 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 alarming sched hrtimer based on total number of SCHED_FAIR tasks a CPU has across its various cfs_rqs.
Cc: Ingo Molnar <mingo@redhat.com> Cc: Peter Zijlstra <peterz@infradead.org> Cc: linux-kernel@vger.kernel.org Signed-off-by: Srivatsa Vaddagiri <vatsa@codeaurora.org> Signed-off-by: Joonwoo Park <joonwoop@codeaurora.org> ---
joonwoop: Do we also need to update or remove if-statement inside hrtick_update()? I guess not because hrtick_update() doesn't want to start hrtick when cfs_rq has large number of nr_running where slice is longer than sched_latency.
kernel/sched/fair.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index 4088eed..c55c566 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -4458,7 +4458,7 @@ 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; -- The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum, hosted by The Linux Foundation
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