linux-kernel - Re: [PATCH 08/15] sched,fair: simplify timeslice length code

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Date:   Thu, 29 Aug 2019 12:00:31 -0400
From:   Rik van Riel <riel@...riel.com>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     linux-kernel <linux-kernel@...r.kernel.org>,
        Kernel Team <kernel-team@...com>, Paul Turner <pjt@...gle.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Mel Gorman <mgorman@...hsingularity.net>
Subject: Re: [PATCH 08/15] sched,fair: simplify timeslice length code

On Thu, 2019-08-29 at 16:02 +0200, Vincent Guittot wrote:
> On Thu, 29 Aug 2019 at 01:19, Rik van Riel <riel@...riel.com> wrote:
> 
> > What am I overlooking?
> 
> My point is more for task that runs several ticks in a row. Their
> sched_slice will be shorter in some cases with your changes so they
> can be preempted earlier by other runnable tasks with a lower
> vruntime
> and there will be more context switch

I can think of exactly one case where the time slice
will be shorter with my new code than with the old code,
and that is the case where:
- A CPU has nr_running > sched_nr_latency
- __sched_period returns a value larger than sysctl_sched_latency
- one of the tasks is much higher priority than the others
- that one task alone gets a timeslice larger than sysctl_sched_latency

With the new code, that high priority task will get a time
slice that is a (large) fraction of sysctl_sched_latency,
while the other (lower priority) tasks get their time slices
rounded up to sysctl_sched_min_granularity.

When tasks get their timeslice rounded up, that will increase
the total sched period in a similar way the old code did by
returning a longer period from __sched_period.

If a CPU is faced with a large number of equal priority tasks,
both the old code and the new code would end up giving each
task a timeslice length of sysctl_sched_min_granularity.

What am I missing?

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