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Message-ID: <1423056858.18770.14.camel@tkhai>
Subject: Re: [PATCH] sched/deadline: Add update_rq_clock() in yield_task_dl()
From: Kirill Tkhai <ktkhai@parallels.com>
To: Xunlei Pang <pang.xunlei@linaro.org>
CC: Peter Zijlstra <peterz@infradead.org>, Juri Lelli <juri.lelli@gmail.com>,
        lkml <linux-kernel@vger.kernel.org>
Date: Wed, 4 Feb 2015 16:34:18 +0300
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Hi, Xunlei,

В Ср, 04/02/2015 в 21:28 +0800, Xunlei Pang пишет:
> I've also sent out this similar patch to yours before,
> and I agree with you on this point :-)

I missed your patch, thanks for the pointing to it.

> But with the latest modification which peter made,
> ( see: cebde6d681aa45f96111cfcffc1544cf2a0454ff )
> I think it would be better to add a skip operation
> after update_curr_dl(rq) like below:
> 
> ---
>  kernel/sched/deadline.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> index e5db8c6..5648e62 100644
> --- a/kernel/sched/deadline.c
> +++ b/kernel/sched/deadline.c
> @@ -915,7 +915,11 @@ static void yield_task_dl(struct rq *rq)
>                 rq->curr->dl.dl_yielded = 1;
>                 p->dl.runtime = 0;
>         }
> +
> +       update_rq_clock(rq);
>         update_curr_dl(rq);
> +       /* Will go to schedule(), to avoid another clock update. */
> +       rq_clock_skip_update(rq, true);
>  }
> 
> #ifdef CONFIG_SMP
> --
> 1.9.1
> 
> On 19 December 2014 at 00:31, Xunlei Pang <pang.xunlei@linaro.org> wrote:
> > yield_task_dl() calls update_curr_dl() which calls start_dl_timer()
> > to throttle current task. But yield_task_dl() doesn't update the rq
> > clock which will cause start_dl_timer() to set the wrong dl_timer
> > which may be much later than current deadline time.
> >
> > For instance, in systems with 100HZ tick, if there're few dl-tasks,
> > then rq clock will mainly depend on the tick to update its value.
> > If we choose the CONFIG_NO_HZ_FULL_ALL feature, things may get even
> > worse.
> >
> > Moreover, there're some statistics in update_curr_dl() also relying
> > on rq clock, like rq_clock_task().
> >
> > Thus, in yield_task_dl(), we should add update_rq_clock() before
> > update_curr_dl().
> >
> > Signed-off-by: Xunlei Pang <pang.xunlei@linaro.org>
> > ---
> >  kernel/sched/deadline.c | 4 ++++
> >  1 file changed, 4 insertions(+)
> >
> > diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> > index e5db8c6..5648e62 100644
> > --- a/kernel/sched/deadline.c
> > +++ b/kernel/sched/deadline.c
> > @@ -915,7 +915,11 @@ static void yield_task_dl(struct rq *rq)
> >                 rq->curr->dl.dl_yielded = 1;
> >                 p->dl.runtime = 0;
> >         }
> > +
> > +       update_rq_clock(rq);
> >         update_curr_dl(rq);
> > +       /* Will go to schedule(), to avoid another clock update. */
> > +       rq->skip_clock_update = 1;
> >  }
> >
> >  #ifdef CONFIG_SMP
> > --
> > 1.9.1
> >

Kirill