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Message-ID: <2a87463e8a51c34733e9c1fcf63380f9caa7afc4.camel@surriel.com>
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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