linux-kernel - Re: [PATCH v5 6/7] sched/deadline: Deferrable dl server

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Date:   Wed, 8 Nov 2023 02:42:16 +0000
From:   Joel Fernandes <joel@...lfernandes.org>
To:     Daniel Bristot de Oliveira <bristot@...nel.org>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Valentin Schneider <vschneid@...hat.com>,
        linux-kernel@...r.kernel.org,
        Luca Abeni <luca.abeni@...tannapisa.it>,
        Tommaso Cucinotta <tommaso.cucinotta@...tannapisa.it>,
        Thomas Gleixner <tglx@...utronix.de>,
        Vineeth Pillai <vineeth@...byteword.org>,
        Shuah Khan <skhan@...uxfoundation.org>,
        Phil Auld <pauld@...hat.com>
Subject: Re: [PATCH v5 6/7] sched/deadline: Deferrable dl server

On Tue, Nov 07, 2023 at 12:58:48PM +0100, Daniel Bristot de Oliveira wrote:
[...]
> >> One more consideration I guess is, because the server is throttled
> >> till 0-laxity time, it is possible that if CFS sleeps even a bit
> >> (after the DL-server is unthrottled), then it will be pushed out to a
> >> full current deadline + period due to CBS. In such a situation,  if
> >> CFS-server is the only DL task running, it might starve RT for a bit
> >> more time.
> >>
> >> Example, say CFS runtime is 0.3s and period is 1s. At 0.7s, 0-laxity
> >> timer fires. CFS runs for 0.29s, then sleeps for 0.005s and wakes up
> >> at 0.295s (its remaining runtime is 0.01s at this point which is < the
> >> "time till deadline" of 0.005s). Now the runtime of the CFS-server
> >> will be replenished to the full 3s (due to CBS) and the deadline
> >> pushed out. The end result is the total runtime that the CFS-server
> >> actually gets is 0.0595s (though yes it did sleep for 5ms in between,
> >> still that's tiny -- say if it briefly blocked on a kernel mutex).
> > 
> > Blah, I got lost in decimal points. Here's the example again:
> > 
> > Say CFS-server runtime is 0.3s and period is 1s.
> > 
> > At 0.7s, 0-laxity timer fires. CFS runs for 0.29s, then sleeps for
> > 0.005s and wakes up at 0.295s (its remaining runtime is 0.01s at this
> > point which is < the "time till deadline" of 0.005s)
> > 
> > Now the runtime of the CFS-server will be replenished to the full 0.3s
> > (due to CBS) and the deadline
> > pushed out.
> > 
> > The end result is, the total runtime that the CFS-server actually gets
> > is 0.595s (though yes it did sleep for 5ms in between, still that's
> > tiny -- say if it briefly blocked on a kernel mutex). That's almost
> > double the allocated runtime.
> 
> I think I got what you mean, and I think I took for granted that we were
> doing overload control on the replenishment, but it seems that we are not..
> 
> I just got back from a doct appt, I will do a proper reply later today.

Ah ok! Thanks Daniel! And hope the appointment went well.

 - Joel