Re: arm64 s2idle vs. workqueues

[Kevin Hilman <khilman@baylibre.com>]

"Rafael J. Wysocki" <rafael@kernel.org> writes:

> Hi Kevin,
>
> On Thu, Oct 10, 2024 at 2:19 AM Kevin Hilman <khilman@baylibre.com> wrote:
>>
>> Hello,
>>
>> Looking for some pointers/tips on debugging s2idle, and in particular
>> why it is not staying in an idle state as long as expected.
>>
>> I'm attempting to use s2idle on a 4-core, single cluster ARM64 SoC (TI
>> AM62x), which doesn't (yet) have any DT defined idle-states, so is just
>> doing a WFI when idle.
>>
>> I'm doing an 8-second s2idle with RTC wakeup by using:
>>
>>   rtcwake -m freeze -s8
>>
>> and what I see is that 3 of the CPUs stay in their idle state for the
>> full 8 seconds, but one of them keeps waking due to the A53
>> arm_arch_timer firing, and processing misc. workqueue related activity
>> (example work listed below[1].)
>>
>> I realize that these workqueues are not WQ_FREEZABLE, so I don't expect
>> the freezer part of suspend to stop/freeze them.  However, I am a bit
>> surprised to see this non-frozen workqueue activity happening often
>> enough (few times per second) to prevent all 4 CPUs from being idle for
>> long periods at the same time, thus preventing a deeper cluster-idle
>> state.
>>
>> Is there something else I'm missing that is needed to keep these
>> workqueues quiet for longer?  I had assumed that most of this workqueue
>> work would be deferred, and shouldn't need to wakeup a CPU just to run.
>>
>> In case it's helpful I have published a trace.dat from trace-cmd which
>> captures power, sched, irq, timer and workqueue events.  With
>> kernelshark, it's pretty obvious to visualize what's happening: CPU0,1,3
>> are all nicely idle for 8 sec while CPU2 is waking due to the timer and
>> workqueue activity.
>>
>> Any pointers to how to improve this situation, or what else needs to be
>> tweaked here would be greatly appreciated,
>
> It looks like tick_freeze() is not called, which only happens in
> enter_s2idle_proper() that is called from cpuidle_enter_s2idle() if
> there are any idle states with an .enter_s2idle() callback.

Ah, thank you for the pointer, this is indeed what is missing.

> So does the cpuidle driver for this platform provide .enter_s2idle()
> callbacks for any of its idle states?

Unfortunately, no, so that explains my problem.

Thanks,

Kevin