s2idle breaks on machines without cpuidle support

[Sudeep Holla]

On Thu, Feb 09, 2023 at 12:42:17AM +0900, Hector Martin wrote:
> On 08/02/2023 19.35, Sudeep Holla wrote:
> > 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.
> 
> The hardware counters would also keep ticking in "real" s2idle (with
> hypothetical PSCI idle support) and often in full suspend. There is a
> flag for this (CLOCK_SOURCE_SUSPEND_NONSTOP) that is set by default for
> ARM. So this isn't why CLOCK_MONOTONIC isn't stopping; there is
> machinery to make the kernel's view of time stop anyway, it's just not
> being invoked here.
>

Indeed, and I assumed s2idle was designed with those requirements but I
think I may be wrong especially looking at few points you have raised
provide my understanding is aligned with yours.

> This is somewhat orthogonal to the issue of PSCI. These machines can't
> physically support PSCI and KVM at the same time (they do not have EL3),
> so PSCI is not an option. We will be starting a conversation about how
> to provide something "like" PSCI over some other sort of transport to
> solve this soon. So that will "fix" this issue once it's all implemented.
>

All the best for the efforts.

> However, these machines aren't the only ones without PSCI (search for
> "spin-table" in arch/arm64/boot/dts, this isn't new and these aren't the
> first).

Yes I am aware of it and if you see arch/arm64/kernel/smp_spin_table.c
we don't support CPU hotplug or suspend for such a system.

> It seems broken that Linux currently implements s2idle in such a
> way that it violates the userspace clock behavior contract on systems
> without a cpuidle driver (and does so silently, to make it worse).

Just to check if I understand this correctly, are you referring to:
cpuidle_idle_call()->default_idle_call() if cpuidle_not_available()
And the problem is it idles there in wfi but CLOCK_MONOTONIC isn't
stopping as expected by the userspace ? If so, fair enough. If not, I
may be missing to understand something.

> So that should be fixed regardless of whether we end up coming up with a
> PSCI alternative or not for these platforms.

If above understanding is correct, I agree. But not sure what was the
motivation behind the current behaviour.

> There's no fundamental reason why s2idle can't work properly with plain WFI.
>

Fair enough. I hadn't thought much of it before as most of the platforms
I have seen or used have at-least one deeper than WFI state these days.
On arm32, this was common but each platform managed suspend_set_ops
on its own and probably can do the same s2idle_set_ops.

--
Regards,
Sudeep