linux-kernel - Re: [PATCH RFC] sched/fair: fix sudden expiration of cfq quota in put_prev

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Message-ID: <20150407134758.GR24151@twins.programming.kicks-ass.net>
Date:	Tue, 7 Apr 2015 15:47:58 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
Cc:	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
	Ben Segall <bsegall@...gle.com>,
	Roman Gushchin <klamm@...dex-team.ru>
Subject: Re: [PATCH RFC] sched/fair: fix sudden expiration of cfq quota in
 put_prev_task()

On Tue, Apr 07, 2015 at 02:52:51PM +0200, Peter Zijlstra wrote:
> If the above set_next_entity() is indeed the simple one, does the below
> cure things?
> 
> ---
>  kernel/sched/fair.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index fdae26eb7218..df72d61138a8 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -5176,12 +5176,11 @@ pick_next_task_fair(struct rq *rq, struct task_struct *prev)
>  simple:
>  	cfs_rq = &rq->cfs;
>  #endif
> +	put_prev_task(rq, prev);
>  
>  	if (!cfs_rq->nr_running)
>  		goto idle;
>  
> -	put_prev_task(rq, prev);
> -
>  	do {
>  		se = pick_next_entity(cfs_rq, NULL);
>  		set_next_entity(cfs_rq, se);

Bah, that's broken because if we end up going idle pick_next_task_idle()
is going to do put_prev_task() again.

Lemme think a bit more on that.
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