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

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Date:   Sun, 18 Sep 2016 07:28:32 +0800
From:   Wanpeng Li <kernellwp@...il.com>
To:     Joonwoo Park <joonwoop@...eaurora.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Srivatsa Vaddagiri <vatsa@...eaurora.org>,
        Ingo Molnar <mingo@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] sched: Fix SCHED_HRTICK bug leading to late preemption of tasks

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

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@...hat.com>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: linux-kernel@...r.kernel.org
> Signed-off-by: Srivatsa Vaddagiri <vatsa@...eaurora.org>
> Signed-off-by: Joonwoo Park <joonwoop@...eaurora.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
>