linux-kernel - [RFC][PATCH 0/2] cpuidle: Allow states to be disabled by default (was: Re: [PATCH] cpuidle: Consolidate disabled state checks)

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Message-ID: <1688511.GgkECGP1XA@kreacher>
Date:   Tue, 19 Nov 2019 00:04:59 +0100
From:   "Rafael J. Wysocki" <rjw@...ysocki.net>
To:     Len Brown <lenb@...nel.org>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        Linux PM <linux-pm@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Doug Smythies <dsmythies@...us.net>,
        LKML <linux-kernel@...r.kernel.org>
Subject: [RFC][PATCH 0/2] cpuidle: Allow states to be disabled by default (was: Re: [PATCH] cpuidle: Consolidate disabled state checks)

On Monday, November 18, 2019 12:26:57 PM CET Rafael J. Wysocki wrote:
> On Mon, Nov 18, 2019 at 10:22 AM Rafael J. Wysocki <rafael@...nel.org> wrote:
> >
> > On Mon, Nov 18, 2019 at 5:46 AM Len Brown <lenb@...nel.org> wrote:
> > >
> > > On Mon, Nov 4, 2019 at 6:16 AM Rafael J. Wysocki <rjw@...ysocki.net> wrote:
> > > >
> > > > From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> > > >
> > > > There are two reasons why CPU idle states may be disabled: either
> > > > because the driver has disabled them or because they have been
> > > > disabled by user space via sysfs.
> > > >
> > > > In the former case, the state's "disabled" flag is set once during
> > > > the initialization of the driver and it is never cleared later (it
> > > > is read-only effectively).
> > >
> > > for x86 (intel_idle and acpi_idle), no states with disabled=1 are  registered
> > > with cpuidle.  Instead, intel_idle (currently) skips them in the loop
> > > that registers states.
> > > (and acpi_idle never touches the disabled field)
> > >
> > > And so for x86, governors checking for drv->states[i].disabled is a NOP,
> > > and the condition described by CPUIDLE_STATE_DISABLED_BY_DRIVER
> > > does not (yet) exist.
> >
> > OK
> >
> > > Looking at the ARM code, it seems that cpuidle-imx6q.c and cpuidle-tegra20.c
> > > reach into the cpuidle states at run time and toggle the
> > > drv->states[i].disabled.
> >
> > I might have overlooked that, let me check.
> >
> > > It seems that this patch takes the initial value of
> > > drv->states->disabled, and sets the (per cpu)
> > > usage.disable=..BY_DRIVER,
> > > but that subsequent run-time toggles in drv->states[i]disabled by
> > > these drivers would be missed,
> > > because you're removed the run-time checking of drv->states->disabled?
> >
> > If it is updated at run time, then yes, the updates will be missed, so
> > thanks for pointing that out.
> >
> > > Finally, I'd like to change intel_idle so that it *can* register a
> > > state that is disabled, by default.
> > > If I change the driver to NOT skip registering disabled states, and
> > > the cpuidle copy has cpuidle_state.disabled=1,
> > > then the state is indeed, unused at run-time.  But as you said,
> > > it is effectively read-only, and is not indicated in sysfs, and can
> > > not be changed via sysfs.
> > >
> > > One way to do this is to do what you do here and initialize
> > > usage.disabled to drv->state.disabled. (not distinguishing between
> > > DRIVER and USER)
> > > That way the user could later over-ride what a driver set, by clearing
> > > the disabled attribute.
> 
> I'd rather get rid of the "disabled" field from struct cpuidle_state
> entirely and introduce a new state flag to indicate the "disabled by
> default" status.
> 
> I also would expose that new flag in a new sysfs attribute of idle
> states, say "disable_default".
> 
> Then, the DISABLED_BY_DRIVER bit would be reserved for driver quirks
> (as per https://patchwork.kernel.org/patch/11249519/) and the
> DISABLED_BY_USER one could be used for all of the other purposes.

To that end, I have the following two experimental patches (on top of
https://patchwork.kernel.org/patch/11249519/) that IMO are simple
enough.

Please let me know what you think.