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

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Date:   Tue, 7 Nov 2023 12:35:40 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Joel Fernandes <joel@...lfernandes.org>
Cc:     Daniel Bristot de Oliveira <bristot@...nel.org>,
        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>,
        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, 7 Nov 2023 11:47:32 -0500
Steven Rostedt <rostedt@...dmis.org> wrote:

> Let me see if I understand what you are asking. By pushing the execution of
> the CFS-server to the end of its period, if it it was briefly blocked and
> was not able to consume all of its zerolax time, its bandwidth gets
> refreshed. Then it can run again, basically doubling its total time.
> 
> But this is basically saying that it ran for its runtime at the start of
> one period and at the beginning of another, right?
> 
> Is that an issue? The CFS-server is still just consuming it's time per
> period. That means that an RT tasks was starving the system that much to
> push it forward too much anyway. I wonder if we just document this
> behavior, if that would be enough?

I may have even captured this scenario.

I ran my migrate[1] program which I use to test RT migration, and it kicks
off a bunch of RT tasks. I like this test because with the
/proc/sys/kernel/sched_rt_* options set, it shows the lines where they are
throttled really well.

This time, I disabled those, and just kept the default:

~# cat /sys/kernel/debug/sched/rq/cpu0/fair_server_defer
1

~# cat /sys/kernel/debug/sched/rq/cpu0/fair_server_period 
1000000000

~# cat /sys/kernel/debug/sched/rq/cpu0/fair_server_runtime 
50000000

And ran my userspin[2] program. And recorded it with:

  trace-cmd record -e sched_switch

The kernelshark output shows the delay from userspin taking up 0.1 seconds
(double the time usually given), with a little preemption in between.

-- Steve

Download attachment "userspin.png" of type "image/png" (307442 bytes)