linux-kernel - Re: [PATCH] sched: Optimize pick_next_task for idle_sched

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Date:   Wed, 1 Mar 2017 11:19:22 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Pavan Kondeti <pkondeti@...eaurora.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] sched: Optimize pick_next_task for idle_sched_class too

On Wed, 1 Mar 2017 17:03:52 +0100
Peter Zijlstra <peterz@...radead.org> wrote:

> On Wed, Mar 01, 2017 at 10:53:03AM -0500, Steven Rostedt wrote:
> > Peter, do we have a solution for this yet? Are you going to add the one
> > with the linker magic?  
> 
> I queued the below earlier today.

Isn't this pretty much identical to the patch I sent you a month ago?

  http://lkml.kernel.org/r/20170119101703.2abeaeb6@gandalf.local.home

-- Steve

> 
> ---
> Subject: sched: Fix pick_next_task() for RT,DL
> From: Peter Zijlstra <peterz@...radead.org>
> Date: Wed Mar  1 10:51:47 CET 2017
> 
> Pavan noticed that commit 49ee576809d8 ("sched/core: Optimize
> pick_next_task() for idle_sched_class") broke RT,DL balancing by
> robbing them of the opportinty to do new-'idle' balancing when their
> last runnable task (on that runqueue) goes away.
> 
> Cc: Steven Rostedt <rostedt@...dmis.org>
> Reported-by: Pavan Kondeti <pkondeti@...eaurora.org>
> Fixes: 49ee576809d8 ("sched/core: Optimize pick_next_task() for idle_sched_class")
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> ---
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -3273,10 +3273,15 @@ pick_next_task(struct rq *rq, struct tas
>  	struct task_struct *p;
>  
>  	/*
> -	 * Optimization: we know that if all tasks are in
> -	 * the fair class we can call that function directly:
> +	 * Optimization: we know that if all tasks are in the fair class we can
> +	 * call that function directly, but only if the @pref task wasn't of a
> +	 * higher scheduling class, because otherwise those loose the
> +	 * opportunity to pull in more work from other CPUs.
>  	 */
> -	if (likely(rq->nr_running == rq->cfs.h_nr_running)) {
> +	if (likely((prev->sched_class == &idle_sched_class ||
> +		    prev->sched_class == &fair_sched_class) &&
> +		   rq->nr_running == rq->cfs.h_nr_running)) {
> +
>  		p = fair_sched_class.pick_next_task(rq, prev, rf);
>  		if (unlikely(p == RETRY_TASK))
>  			goto again;