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From:   Vincent Guittot <vincent.guittot@linaro.org>
Date:   Thu, 19 May 2022 15:55:25 +0200
Message-ID: <CAKfTPtCx_fhud-VsxCi9jS==yGavn7zgRBQyUfzXgbTiX2+77g@mail.gmail.com>
Subject: Re: [PATCH v2 5/7] sched/fair: Take into account latency nice at wakeup
To:     Josh Don <joshdon@google.com>
Cc:     Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Juri Lelli <juri.lelli@redhat.com>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Benjamin Segall <bsegall@google.com>,
        Mel Gorman <mgorman@suse.de>,
        Daniel Bristot de Oliveira <bristot@redhat.com>,
        Valentin Schneider <vschneid@redhat.com>,
        linux-kernel <linux-kernel@vger.kernel.org>, parth@linux.ibm.com,
        Qais Yousef <qais.yousef@arm.com>,
        "Hyser,Chris" <chris.hyser@oracle.com>,
        Valentin Schneider <valentin.schneider@arm.com>,
        patrick.bellasi@matbug.net, David.Laight@aculab.com,
        Paul Turner <pjt@google.com>, pavel@ucw.cz,
        Tejun Heo <tj@kernel.org>, Quentin Perret <qperret@google.com>,
        Tim Chen <tim.c.chen@linux.intel.com>
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On Tue, 17 May 2022 at 02:54, Josh Don <joshdon@google.com> wrote:
>
> Hi Vincent,
>
> On Thu, May 12, 2022 at 9:36 AM Vincent Guittot
> <vincent.guittot@linaro.org> wrote:
> >
> > Take into account the nice latency priority of a thread when deciding to
> > preempt the current running thread. We don't want to provide more CPU
> > bandwidth to a thread but reorder the scheduling to run latency sensitive
> > task first whenever possible.
> >
> > As long as a thread didn't use its bandwidth, it will be able to preempt
> > the current thread.
> >
> > At the opposite, a thread with a low latency priority will preempt current
> > thread at wakeup only to keep fair CPU bandwidth sharing. Otherwise it will
> > wait for the tick to get its sched slice.
>
> Following up from the discussion on the prior series, I'm still not
> sure why this approach is exclusive of extending the entity placement
> code; I think both changes together would be useful.
>
> By only changing the wakeup preemption decision, you're only
> guaranteeing that the latency-sensitive thing on cpu won't be
> preempted until the next sched tick, which can occur at any time
> offset from the wakeup, from 0ns to the length of one tick. If a

In fact, you are ensured to run a minimum time of 3ms at least on >=8
cores system before tick can preempt you. I considered updating this
part as well to increase the value for negative weight but didn't do
it for now. I can have a look

> latency-tolerant task wakes up with a lot of sleeper credit, it would
> pretty quickly preempt a latency-sensitive task on-cpu, even if it
> doesn't initially do so due to the above changes to wakeup preemption.
>
> Adjusting place_entity wouldn't impact cpu bandwidth in steady-state
> competition between threads of different latency prio, it would only
> impact slightly at wakeup, in a similar but more consistent manner to
> the changes above to wakeup_preempt_entity (ie. a task that is not
> latency sensitive might have to wait a few ticks to preempt a latency
> sensitive task, even if it was recently sleeping for a while).