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Date: Mon, 27 Feb 2017 12:54:30 -0500
From: Steven Rostedt <rostedt@goodmis.org>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Pavan Kondeti <pkondeti@codeaurora.org>,
        LKML <linux-kernel@vger.kernel.org>, Ingo Molnar <mingo@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [PATCH] sched: Optimize pick_next_task for idle_sched_class too
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Sorry, for the late reply. Just got back from traveling.

On Thu, 23 Feb 2017 18:54:38 +0100
Peter Zijlstra <peterz@infradead.org> wrote:

> On Thu, Feb 23, 2017 at 06:45:05PM +0100, Peter Zijlstra wrote:
> > Hurm.. maybe we should do what Steve initially suggested. The
> > alternative is link order trickery, and I'm not sure we want to do that.  
> 
> That is, given:
> 
> kernel/sched/Makefile: obj-y += idle_task.o fair.o rt.o deadline.o stop_task.o
> 
> results in:
> 
> readelf -s defconfig-build/vmlinux | awk '/sched_class/ {print $2 " " $8}' | sort -n
> 00000000602c93c0 idle_sched_class
> 00000000602c9480 fair_sched_class
> 00000000602c9580 rt_sched_class
> 00000000602c96c0 dl_sched_class
> 00000000602c97c0 stop_sched_class
> 
> we can do this, but yuck!
> 
> ---
>  kernel/sched/core.c | 12 +++++++++---
>  1 file changed, 9 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 8f972df76eb2..eebe6729ceb7 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -3285,10 +3285,16 @@ pick_next_task(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
>  	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 @prev task wasn't of a
> +	 * higher scheduling class, because otherwise those loose the
> +	 * opportinity to pull in more work from other CPUs.
> +	 *
> +	 * Depends on link order in kernel/sched/Makefile.
>  	 */
> -	if (likely(rq->nr_running == rq->cfs.h_nr_running)) {
> +	if (likely(rq->nr_running == rq->cfs.h_nr_running &&
> +		   prev->sched_class <= &fair_sched_class)) {

If we go this route, I would suggest that we hardcode the classes in
vmlinux.lds.h.

-- Steve

> +
>  		p = fair_sched_class.pick_next_task(rq, prev, rf);
>  		if (unlikely(p == RETRY_TASK))
>  			goto again;