linux-kernel - Re: [PATCH v5 4/4] sched/fair: update util_est only on util

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Message-ID: <20180301174652.GB26235@e110439-lin>
Date:   Thu, 1 Mar 2018 17:46:52 +0000
From:   Patrick Bellasi <patrick.bellasi@....com>
To:     linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org
Cc:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Paul Turner <pjt@...gle.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Todd Kjos <tkjos@...roid.com>,
        Joel Fernandes <joelaf@...gle.com>,
        Steve Muckle <smuckle@...gle.com>
Subject: Re: [PATCH v5 4/4] sched/fair: update util_est only on util_avg
 updates

The changelog is missing the below CCs. :(

Since that's a new patch in this series, I expect some feedbacks and
thus I'll add them on the next respin.

On 22-Feb 17:01, Patrick Bellasi wrote:
> The estimated utilization of a task is currently updated every time the
> task is dequeued. However, to keep overheads under control, PELT signals
> are effectively updated at maximum once every 1ms.
> 
> Thus, for really short running tasks, it can happen that their util_avg
> value has not been updates since their last enqueue.  If such tasks are
> also frequently running tasks (e.g. the kind of workload generated by
> hackbench) it can also happen that their util_avg is updated only every
> few activations.
> 
> This means that updating util_est at every dequeue potentially introduces
> not necessary overheads and it's also conceptually wrong if the util_avg
> signal has never been updated during a task activation.
> 
> Let's introduce a throttling mechanism on task's util_est updates
> to sync them with util_avg updates. To make the solution memory
> efficient, both in terms of space and load/store operations, we encode a
> synchronization flag into the LSB of util_est.enqueued.
> This makes util_est an even values only metric, which is still
> considered good enough for its purpose.
> The synchronization bit is (re)set by __update_load_avg_se() once the
> PELT signal of a task has been updated during its last activation.
> 
> Such a throttling mechanism allows to keep under control util_est
> overheads in the wakeup hot path, thus making it a suitable mechanism
> which can be enabled also on high-intensity workload systems.
> Thus, this now switches on by default the estimation utilization
> scheduler feature.
> 
> Suggested-by: Chris Redpath <chris.redpath@....com>
> Signed-off-by: Patrick Bellasi <patrick.bellasi@....com>

Cc: Ingo Molnar <mingo@...hat.com>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Paul Turner <pjt@...gle.com>
Cc: Vincent Guittot <vincent.guittot@...aro.org>
Cc: Morten Rasmussen <morten.rasmussen@....com>
Cc: Dietmar Eggemann <dietmar.eggemann@....com>
Cc: linux-kernel@...r.kernel.org

[...]

-- 
#include <best/regards.h>

Patrick Bellasi