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Date:   Tue, 10 Oct 2023 10:08:40 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Youssef Esmat <youssefesmat@chromium.org>
Cc:     Daniel Jordan <daniel.m.jordan@oracle.com>, mingo@kernel.org,
        vincent.guittot@linaro.org, linux-kernel@vger.kernel.org,
        juri.lelli@redhat.com, dietmar.eggemann@arm.com,
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        tglx@linutronix.de
Subject: Re: [PATCH 00/15] sched: EEVDF and latency-nice and/or slice-attr
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On Thu, Oct 05, 2023 at 01:23:14PM -0500, Youssef Esmat wrote:
> On Thu, Oct 5, 2023 at 7:06 AM Peter Zijlstra <peterz@infradead.org> wrote:

> > BAaaBCccccccccBBBAaaBBBAaaBB                            BBAaaBBBAaaBBBAaaBCccccccccB
> >
> >
> >
> > As I wrote before; EVDF has worse lag bounds, but this is not
> > insurmountable. The biggest problem that I can see is that of wakeup
> > preemption. Currently we allow to preempt when 'current' has reached V
> > (RUN_TO_PARITY in pick_eevdf()).
> >
> > With these rules, when EEVDF schedules C (our large slice task) at t=10
> > above, it is only a little behind C and can be reaily preempted after
> > about 2 time units.
> >
> > However, EVDF will delay scheduling C until much later, see how A and B
> > walk far ahead of V until t=36. Only when will we pick C. But this means
> > that we're firmly stuck with C for at least 11 time units. A newly
> > placed task will be around V and will have no chance to preempt.
> >
> 
> Thank you for the detailed analysis! I am still in the process of
> digesting everything.
> I do have a quick question, this will only be the case if we adjust
> C's runtime without adjusting nice value, correct? So it does not
> currently apply to the submitted code where the only way to change the
> deadline is to also change the nice value and thus how fast/slow
> vruntime accumulates. In other words without the sched_runtime
> patch[1] we should not run into this scenario, correct?
> 
> [1] https://lore.kernel.org/lkml/20230915124354.416936110@noisy.programming.kicks-ass.net/


Much harder to run into it, but you can still hit it using nice.

 d_i = v_i + r_i / w_i

So for very light tasks, the effective v_deadline goes out lots.

And as I wrote yesterday, by delaying scheduling (far) past 0-lag you
effectively open up a denial of service window, since new tasks are
placed around 0-lag.

Now, nobody will likely care much if very light tasks get horrific
treatment, but still, it's there.