Re: [PATCH v2] PM: runtime: avoid priority inversion on PREEMPT_RT

[John Keeping]

On Wed, 17 Nov 2021 19:53:47 +0100
"Rafael J. Wysocki" <rafael@xxxxxxxxxx> wrote:

> On Wed, Nov 17, 2021 at 7:37 PM John Keeping <john@xxxxxxxxxxxx> wrote:
> >
> > With PREEMPT_RT the cpu_relax() loops in rpm_suspend and rpm_resume can
> > cause unbounded latency if they preempt an asynchronous suspend.  The
> > main scenario where this can happen is when a realtime thread resumes a
> > device while it is asynchronously suspending on a worker thread.
> >
> > I'm not convinced this can actually happen in the rpm_suspend case, or
> > at least it's a lot less likely for a synchronous suspend to run at the
> > same time as an asynchronous suspend, but both functions are updated
> > here for symmetry.
> >
> > For devices setting power.irq_safe, it is possible that RPM functions
> > will be called with a spinlock held (for example in
> > pl330_issue_pending()).  This means a normal call to schedule() can't be
> > used, but to avoid the priority inversion it is necessary to wait and
> > schedule.  schedule_rtlock() is only available when CONFIG_PREEMPT_RT is
> > defined, so even though the logic is correct without any preprocessor
> > guards around schedule_rtlock(), they are necessary for compilation.
> >
> > Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> > Cc: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> > Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> > Signed-off-by: John Keeping <john@xxxxxxxxxxxx>
> > ---
> > Changes since v1:
> > - Use schedule_rtlock() instead of schedule() for PREEMPT_RT & irq_safe
> > - Rewritten commit description
> >
> >  drivers/base/power/runtime.c | 18 ++++++++++++++----
> >  1 file changed, 14 insertions(+), 4 deletions(-)
> >
> > diff --git a/drivers/base/power/runtime.c b/drivers/base/power/runtime.c
> > index f3de7bfc7f5b..fdf461bfae8c 100644
> > --- a/drivers/base/power/runtime.c
> > +++ b/drivers/base/power/runtime.c
> > @@ -596,7 +596,7 @@ static int rpm_suspend(struct device *dev, int rpmflags)
> >                         goto out;
> >                 }
> >
> > -               if (dev->power.irq_safe) {
> > +               if (dev->power.irq_safe && !IS_ENABLED(CONFIG_PREEMPT_RT)) {  
> 
> Please add a helper to avoid code duplication related to this (even
> though there is a small amount of it).

Ack.  I'd like some feedback on the schedule_rtlock() approach from the
scheduler & RT people, so I'll wait a bit before sending a v3.