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

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Date:   Thu, 25 Nov 2021 19:43:42 +0530
From:   Maulik Shah <quic_mkshah@...cinc.com>
To:     Peter Zijlstra <peterz@...radead.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Daniel Lezcano <daniel.lezcano@...aro.org>
CC:     <bjorn.andersson@...aro.org>, <rafael@...nel.org>,
        <daniel.lezcano@...aro.org>, <linux-arm-msm@...r.kernel.org>,
        <linux-pm@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <ulf.hansson@...aro.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

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.

We can not take PM-QoS from driver to prevent deep cpuidle since all the 
vendor modules are kept in a separate partition and will be loaded only 
after kernel boot up is done
and by this time kernel already starts executing deep cpuidle modes.
>
> Surely the better solution is to rework things to not rely on this. I'm
> fairly sure it's not hard to write a cpuidle driver that does much the
> same.
The other option i think is to pass cpuidle.off=1 in kernel command line 
and then add enable_cpuidle() in drivers/cpuidle/cpuidle.c
something similar as below which can be called by vendor module.

void enable_cpuidle(void)
{
         off = 0;
}
EXPORT_SYMBOL_GPL(enable_cpuidle);

This may be a good option since we have already disable_cpuidle() but 
not enable_cpuidle().

void disable_cpuidle(void)
{
         off = 1;
}

Hi Rafael/Daniel, can you please let me know your suggestion on 
this/similar implementation?

Thanks,
Maulik