linux-kernel - Re: [PATCH] sched/deadline: dome some cleanup for push_dl

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Date:   Sun, 26 Jul 2020 22:30:13 +0800
From:   Peng Liu <iwtbavbm@...il.com>
To:     Juri Lelli <juri.lelli@...hat.com>
Cc:     mingo@...hat.com, peterz@...radead.org, vincent.guittot@...aro.org,
        dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
        mgorman@...e.de, bristot@...hat.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sched/deadline: dome some cleanup for push_dl_task()

On Fri, Jul 24, 2020 at 09:14:26AM +0200, Juri Lelli wrote:
> Hi,
> 
> On 07/07/20 00:04, Peng Liu wrote:
> > 'commit 840d719604b0 ("sched/deadline: Update rq_clock of later_rq when pushing a task")'
> > introduced the update_rq_clock() to fix the "used-before-update" bug.
> > 
> > 'commit f4904815f97a ("sched/deadline: Fix double accounting of rq/running bw in push & pull")'
> > took away the bug source(add_running_bw()).
> > 
> > We no longer need to update rq_clock in advance, let activate_task()
> > worry about that.
> > 
> > Signed-off-by: Peng Liu <iwtbavbm@...il.com>
> > ---
> >  kernel/sched/deadline.c | 8 +-------
> >  1 file changed, 1 insertion(+), 7 deletions(-)
> > 
> > diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> > index 504d2f51b0d6..c3fa11f84d93 100644
> > --- a/kernel/sched/deadline.c
> > +++ b/kernel/sched/deadline.c
> > @@ -2104,13 +2104,7 @@ static int push_dl_task(struct rq *rq)
> >  
> >  	deactivate_task(rq, next_task, 0);
> >  	set_task_cpu(next_task, later_rq->cpu);
> > -
> > -	/*
> > -	 * Update the later_rq clock here, because the clock is used
> > -	 * by the cpufreq_update_util() inside __add_running_bw().
> > -	 */
> > -	update_rq_clock(later_rq);
> > -	activate_task(later_rq, next_task, ENQUEUE_NOCLOCK);
> > +	activate_task(later_rq, next_task, 0);
> >  	ret = 1;
> 
> The change looks good to me, since now add_running_bw() is called later
> by enqueue_task_dl(), but rq_clock has already been updated by core's
> enqueue_task().
> 

Thanks for your time.

> Daniel, Dietmar, a second pair of eyes (since you authored the commits
> above)?
> 
> I'd chage subject to something like "sched/deadline: Stop updating
> rq_clock before pushing a task".
> 
> Thanks,
> 
> Juri
>