linux-kernel - Re: [PATCH 0/6] cpufreq: schedutil: fixes for flags updates

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Date:   Thu, 2 Mar 2017 17:09:20 +0100
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Patrick Bellasi <patrick.bellasi@....com>
Cc:     linux-kernel <linux-kernel@...r.kernel.org>,
        "linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        John Stultz <john.stultz@...aro.org>,
        Juri Lelli <juri.lelli@....com>, Todd Kjos <tkjos@...roid.com>,
        Tim Murray <timmurray@...gle.com>,
        Andres Oportus <andresoportus@...gle.com>,
        Joel Fernandes <joelaf@...gle.com>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Chris Redpath <chris.redpath@....com>
Subject: Re: [PATCH 0/6] cpufreq: schedutil: fixes for flags updates

On 2 March 2017 at 16:45, Patrick Bellasi <patrick.bellasi@....com> wrote:
> The current version of schedutil has some issues related to the management
> of update flags used by systems with frequency domains spawning multiple CPUs.
>
> Each time a CPU utilisation update is issued by the scheduler a set of flags
> are configured to define (mainly) which class is asking for a utilisation
> update. These flags are then used by the frequency selection policy to
> identify the OPP to choose.
>
> In the current implementation, CPU flags are overridden each time the
> scheduler calls schedutil for an update. Such a behaviour produces issues
> in these scenarios, where we assume CPU1 and CPU2 share the same frequency
> domain:
> a) a RT task which executed on CPU1 can keep the domain at an high frequency
>    for a long period of time, even if there are no longer RT tasks on
>    CPUs in that domain

Normally this is dropped after a tick.
Nevertheless, there is an issue if the freq_update_delay_ns is shorter
than a tick because of sugov RT thread

> b) a FAIR task co-scheduled in the same CPU of a RT task can override the
>    flags configured by the RT task and potentially this can cause an
>    unwanted frequency drop
>
> These misbehaviours have been verified using a set of simple rt-app based
> synthetic workloads, running on a ARM's Juno board, and results are
> available in this Notebook [1].
>
> This series proposes a set of fixes for the aforementioned issues as well
> as a small improvement to speedup the selection of the maximum frequency
> when RT tasks enter a CPU.
>
> This series is based on top of today's tip/sched/core and is public available
> from this repository:
>
>   git://www.linux-arm.com/linux-pb eas/schedutil/flags_fixes
>
> Cheers Patrick
>
> [1] https://gist.github.com/d6a21b459a18091b2b058668a550010d
>
> Patrick Bellasi (6):
>   cpufreq: schedutil: reset sg_cpus's flags at IDLE enter
>   cpufreq: schedutil: ignore the sugov kthread for frequencies
>     selections
>   cpufreq: schedutil: ensure max frequency while running RT/DL tasks
>   cpufreq: schedutil: relax rate-limiting while running RT/DL tasks
>   cpufreq: schedutil: avoid utilisation update when not necessary
>   sched/rt: fast switch to maximum frequency when RT tasks are scheduled
>
>  include/linux/sched.h            |  1 +
>  kernel/sched/cpufreq_schedutil.c | 59 ++++++++++++++++++++++++++++++++++------
>  kernel/sched/idle_task.c         |  4 +++
>  kernel/sched/rt.c                | 15 ++++++++--
>  4 files changed, 68 insertions(+), 11 deletions(-)
>
> --
> 2.7.4
>