linux-kernel - Re: [PATCH] sched/idle: Export cpu_idle_poll

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Message-ID: <CAPDyKFo8Z+G-14t-o-HiHf1_xuexBAGo3eumca-0QO8eTqNpsw@mail.gmail.com>
Date:   Fri, 26 Nov 2021 13:05:12 +0100
From:   Ulf Hansson <ulf.hansson@...aro.org>
To:     Maulik Shah <quic_mkshah@...cinc.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        bjorn.andersson@...aro.org, linux-arm-msm@...r.kernel.org,
        linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org,
        quic_lsrao@...cinc.com, rnayak@...eaurora.org,
        Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>
Subject: Re: [PATCH] sched/idle: Export cpu_idle_poll_ctrl() symbol

On Thu, 25 Nov 2021 at 15:13, Maulik Shah <quic_mkshah@...cinc.com> wrote:
>
> Hi Peter,
>
> On 11/25/2021 3:21 PM, Peter Zijlstra wrote:
> > On Thu, Nov 25, 2021 at 02:44:36PM +0530, Maulik Shah wrote:
> >> Export cpu_idle_poll_ctrl() so that module drivers can use same.
> > This does not seem like a really safe interface to expose to the
> > world.
>
> Thanks for the review.
>
> Keeping the cpuidle enabled from boot up may delay/increase the boot up
> time.
> Below is our use case to force cpuidle to stay in cpu_idle_poll().
>
> We keep cpuidle disabled from boot up using "nohlt" option of kernel
> command line which internally sets cpu_idle_force_poll = 1;
> and once the device bootup reaches till certain point (for example the
> android homescreen is up) userspace may notify a
> vendor module driver which can invoke cpu_idle_poll_ctrl(false); to come
> out of poll mode.
> So vendor module driver needs cpu_idle_poll_ctrl() exported symbol.

Waiting for the homescreen can be considered as rather late, from the
kernel boot progress point of view.

That said, I am wondering if a similar improvement can be achieved by
just allowing WFI (thus no deeper idle states) until homescreen? If
so, that can be quite easily achieved by modularizing the cpuidle-psci
driver.

[...]

Kind regards
Uffe