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

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Date:   Mon, 19 Sep 2016 10:21:58 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Joonwoo Park <joonwoop@...eaurora.org>
Cc:     Srivatsa Vaddagiri <vatsa@...eaurora.org>,
        Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sched: Fix SCHED_HRTICK bug leading to late preemption
 of tasks

On Fri, Sep 16, 2016 at 06:28:51PM -0700, Joonwoo Park wrote:
> From: Srivatsa Vaddagiri <vatsa@...eaurora.org>
> 
> SCHED_HRTICK feature is useful to preempt SCHED_FAIR tasks on-the-dot

Right, but I always found the overhead of the thing too high to be
really useful.

How come you're using this?


>  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.

Right, you want that to match with whatever sched_slice() does.

> +++ 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;

Yeah, that looks right. I don't think I've ever tried hrtick with
cgroups enabled...