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

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Date:   Thu, 19 Jan 2017 18:44:08 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     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 Thu, Jan 19, 2017 at 10:17:03AM -0500, Steven Rostedt wrote:
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 154fd68..e2c6d3b 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -3259,13 +3259,15 @@ static inline struct task_struct *
>  pick_next_task(struct rq *rq, struct task_struct *prev, struct pin_cookie cookie)
>  {
>  	const struct sched_class *class = &fair_sched_class;
> +	const struct sched_class *idle_class = &idle_sched_class;
>  	struct task_struct *p;
>  
>  	/*
>  	 * Optimization: we know that if all tasks are in
>  	 * the fair class we can call that function directly:
>  	 */
> -	if (likely(prev->sched_class == class &&
> +	if (likely((prev->sched_class == class ||
> +		    prev->sched_class == idle_class) &&
>  		   rq->nr_running == rq->cfs.h_nr_running)) {

OK, so I hate this patch because it makes the condition more complex,
and while staring at what it does for code generation I couldn't for the
life of me figure out why we care about prev->sched_class to begin with.

(we used to, but the current code not so much)

So I simply removed that entire clause, like below, and lo and behold,
the system booted...

Could you give it a spin to see if anything comes apart?


diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 49ce1cb..51ca21e 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -3321,15 +3321,14 @@ static inline void schedule_debug(struct task_struct *prev)
 static inline struct task_struct *
 pick_next_task(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
 {
-	const struct sched_class *class = &fair_sched_class;
+	const struct sched_class *class;
 	struct task_struct *p;
 
 	/*
 	 * Optimization: we know that if all tasks are in
 	 * the fair class we can call that function directly:
 	 */
-	if (likely(prev->sched_class == class &&
-		   rq->nr_running == rq->cfs.h_nr_running)) {
+	if (likely(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;