linux-kernel - Re: [PATCH] cpuidle: Consolidate disabled state checks

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Date:   Mon, 18 Nov 2019 10:22:05 +0100
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Len Brown <lenb@...nel.org>
Cc:     "Rafael J. Wysocki" <rjw@...ysocki.net>,
        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: Re: [PATCH] cpuidle: Consolidate disabled state checks

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.
>
> However, the ARM drivers, at least, seem to want to reserve the right
> to set and clear the drv->state.disabled,
> and to have them continue to have that right, we have to continue
> checking that field at run-time.

Alternatively, the drivers in question can be changed to update the
disable field in state_usage instead (maybe under a lock to prevent
them from racing with user space).

> And giving drivers the opportunity to do that disabling driver-wide,
> instead of per-cpu (usage) wide,
> seems to be something we may want to keep.

So it looks like you want me to revert this patch which is something
that I really don't want to do, because of the extra checks all over
the place which are simply pointless in the majority of cases.

Cheers,
Rafael