linux-kernel - Re: s2idle breaks on machines without cpuidle support

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Date:   Wed, 8 Feb 2023 10:35:11 +0000
From:   Sudeep Holla <sudeep.holla@....com>
To:     Kazuki <kazukih0205@...il.com>
Cc:     linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Lorenzo Pieralisi <lpieralisi@...nel.org>,
        Hector Martin <marcan@...can.st>,
        Sven Peter <sven@...npeter.dev>,
        Len Brown <len.brown@...el.com>, Pavel Machek <pavel@....cz>
Subject: Re: s2idle breaks on machines without cpuidle support

On Wed, Feb 08, 2023 at 04:48:18AM +0900, Kazuki wrote:
> On Mon, Feb 06, 2023 at 10:12:39AM +0000, Sudeep Holla wrote:
> >
> > What do you mean by break ? More details on the observation would be helpful.
> For example, CLOCK_MONOTONIC doesn't stop even after suspend since
> these chain of commands don't get called.
>
> call_cpuidle_s2idle->cpuidle_enter_s2idle->enter_s2idle_proper->tick_freeze->sched_clock_suspend (Function that pauses CLOCK_MONOTONIC)
>
> Which in turn causes programs like systemd to crash since it doesn't
> expect this.

Yes expected IIUC. The per-cpu timers and counters continue to tick in
WFI and hence CLOCK_MONOTONIC can't stop.

> >
> > > 2. Suspend actually works on ARM64 machines even without proper
> > > cpuidle (PSCI cpuidle) since they support wfi, so the assumption here is wrong
> > > on such machines
> > >
> >
> > Sorry I am bit confused here. Your point (2) contradicts the $subject.
> drivers/cpuidle/cpuidle.c:
>
> bool cpuidle_not_available(struct cpuidle_driver *drv,
> 			   struct cpuidle_device *dev)
> {
> 	return off || !initialized || !drv || !dev || !dev->enabled;
> }
>
> The cpuidle framework reports ARM64 devices without PSCI cpuidle as
> "cpuidle not available" even when they support wfi, which causes suspend
> to fail, which shouldn't be happening since they do support idling.

Yes with just WFI, there will be no active cpuidle driver.

[...]

> > Again, since s2idle is userspace driven, I don't understand what do you
> > mean by unbootable kernel in the context of s2idle.
>
> Sorry, I meant "attempts to fix this bug have all led to an unbootable
> kernel."

Again I assume you mean kernel hang or crash and nothing to do with boot.
Once you enter s2i state with your changes/fix, it hangs or is unresponsive
as it might have either failed to enter or resume from the state.

--
Regards,
Sudeep