Re: [PATCH] sched: Fix SCHED_HRTICK bug leading to late preemption of tasks

[Wanpeng Li]

2016-09-17 9:28 GMT+08:00 Joonwoo Park <joonwoop@xxxxxxxxxxxxxx>:
> From: Srivatsa Vaddagiri <vatsa@xxxxxxxxxxxxxx>
>
> 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

not be preempted much, right?

> 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@xxxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> Signed-off-by: Srivatsa Vaddagiri <vatsa@xxxxxxxxxxxxxx>
> Signed-off-by: Joonwoo Park <joonwoop@xxxxxxxxxxxxxx>
> ---
>
>  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;
> --
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