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Date:   Wed, 6 Sep 2023 22:08:48 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Daniel Bristot de Oliveira <bristot@redhat.com>
Cc:     Daniel Bristot de Oliveira <bristot@kernel.org>,
        Ingo Molnar <mingo@redhat.com>,
        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>,
        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>,
        Joel Fernandes <joel@joelfernandes.org>,
        Vineeth Pillai <vineeth@bitbyteword.org>,
        Shuah Khan <skhan@linuxfoundation.org>,
        Phil Auld <pauld@redhat.com>
Subject: Re: [PATCH v4 6/7] sched/deadline: Deferrable dl server
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On Wed, Sep 06, 2023 at 10:04:06PM +0200, Peter Zijlstra wrote:
> On Wed, Sep 06, 2023 at 04:58:11PM +0200, Daniel Bristot de Oliveira wrote:
> 
> > > So one thing we could do is have update_curr_fair() decrement
> > > fair_server's runtime and yield the period then it hits 0 (and capping
> > > it at 0, not allowing it to go negative or so).
> > > 
> > > That way you only force the situation when FAIR hasn't had it's allotted
> > > time this perio, and only for as much as to make up for the time it
> > > lacks.
> > 
> > We can also decrease the runtime to a negative number while in
> > defer/throttle state, and let the while in replenish_dl_entity() to
> > replenish with the += runtime;
> 
> Yes, but my point was that fair_server gives a lower bound of runtime
> per period, more -- if available -- is fine.
> 
> If we allow negative runtime, you'll affect future periods, and that is
> not desired in this case.
> 
> Or am I still confused?

That is, let update_curr_fair() decrement fair_server runtime
*unconditionally* -- even if the task was not selected through the
server.

Specifically, if the fair task is selected normally due to lack of
deadline tasks, that runtime *still* counts towards the fair-server and
have the server yield the period when zero.

This means that fair_server is only effective IFF 'normal' execution
doesn't match the fair_server.runtime execution.