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

From: Joel Fernandes <joel@joelfernandes.org>
To: Daniel Bristot de Oliveira <bristot@kernel.org>
Cc: Ingo Molnar <mingo@redhat.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Juri Lelli <juri.lelli@redhat.com>,
	Vincent Guittot <vincent.guittot@linaro.org>,
	Dietmar Eggemann <dietmar.eggemann@arm.com>,
	Steven Rostedt <rostedt@goodmis.org>,
	Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
	Daniel Bristot de Oliveira <bristot@redhat.com>,
	Valentin Schneider <vschneid@redhat.com>,
	linux-kernel@vger.kernel.org,
	Luca Abeni <luca.abeni@santannapisa.it>,
	Tommaso Cucinotta <tommaso.cucinotta@santannapisa.it>,
	Thomas Gleixner <tglx@linutronix.de>,
	Vineeth Pillai <vineeth@bitbyteword.org>,
	Shuah Khan <skhan@linuxfoundation.org>,
	Phil Auld <pauld@redhat.com>
Subject: Re: [PATCH v5 6/7] sched/deadline: Deferrable dl server
Date: Wed, 8 Nov 2023 02:42:16 +0000	[thread overview]
Message-ID: <20231108024216.GB2992223@google.com> (raw)
In-Reply-To: <d6811957-1cb7-489d-a3da-f38622d7f515@kernel.org>

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