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

From: Vincent Guittot <vincent.guittot@linaro.org>
To: Rik van Riel <riel@surriel.com>
Cc: linux-kernel <linux-kernel@vger.kernel.org>,
	Kernel Team <kernel-team@fb.com>, Paul Turner <pjt@google.com>,
	Dietmar Eggemann <dietmar.eggemann@arm.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Ingo Molnar <mingo@redhat.com>,
	Morten Rasmussen <morten.rasmussen@arm.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Mel Gorman <mgorman@techsingularity.net>
Subject: Re: [PATCH 08/15] sched,fair: simplify timeslice length code
Date: Fri, 30 Aug 2019 08:41:17 +0200	[thread overview]
Message-ID: <CAKfTPtCAU7bT3sJ_FPexqKrfFzd8Yk0hVTEB5Da=+VbqPViXpA@mail.gmail.com> (raw)
In-Reply-To: <2a87463e8a51c34733e9c1fcf63380f9caa7afc4.camel@surriel.com>

On Thu, 29 Aug 2019 at 18:00, Rik van Riel <riel@surriel.com> wrote:
>
> On Thu, 2019-08-29 at 16:02 +0200, Vincent Guittot wrote:
> > On Thu, 29 Aug 2019 at 01:19, Rik van Riel <riel@surriel.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

yes nr_running must be higher than  sched_nr_latency

> - __sched_period returns a value larger than sysctl_sched_latency
> - one of the tasks is much higher priority than the others

it's not only one, that can be several. It depends of the number of
running tasks

> - 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,

yes

> while the other (lower priority) tasks get their time slices
> rounded up to sysctl_sched_min_granularity.

yes and if the jify period is higher than sysctl_sched_min_granularity
they will get a full jiffy period

>
> 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.

sched_slice is not a strict value and scheduler will not schedule out
the task after the sched_slice (unless you enable HRTICK which is
disable by default). Instead it will wait for next tick to change the
running task

sched_slice is mainly use to ensure a minimum running time in a row.
With this change, the running time of the high priority task will most
probably be split in several slice instead of one

>
> 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?
>
> --
> All Rights Reversed.