Re: [PATCH v4] cpuidle: psci: Do not suspend topology CPUs on PREEMPT_RT

[Ulf Hansson]

On Wed, 25 Jan 2023 at 17:46, Daniel Lezcano <daniel.lezcano@xxxxxxxxxx> wrote:
>
>
> Hi Krzysztof,
>
>
> On 25/01/2023 12:34, Krzysztof Kozlowski wrote:
> > The runtime Power Management of CPU topology is not compatible with
> > PREEMPT_RT:
> > 1. Core cpuidle path disables IRQs.
> > 2. Core cpuidle calls cpuidle-psci.
> > 3. cpuidle-psci in __psci_enter_domain_idle_state() calls
> >     pm_runtime_put_sync_suspend() and pm_runtime_get_sync() which use
> >     spinlocks (which are sleeping on PREEMPT_RT).
> >
> > Deep sleep modes are not a priority of Realtime kernels because the
> > latencies might become unpredictable.  On the other hand the PSCI CPU
> > idle power domain is a parent of other devices and power domain
> > controllers, thus it cannot be simply skipped (e.g. on Qualcomm SM8250).
> >
> > Disable the idle callbacks in cpuidle-psci and mark the domain as
> > always on.  This is a trade-off between making PREEMPT_RT working and
> > still having a proper power domain hierarchy in the system.
>
> Wouldn't make sense to rely on the latency constraint framework ?

The main problem is that for runtime PM there is a per device spinlock
being used, which becomes a sleepable lock on PREEMPT_RT.

In other words, the only simple solution is to avoid the calls to
runtime PM in the idle path.

[...]

Kind regards
Uffe