Re: [PATCH v3 7/7] cpuidle/poll_state: replace cpu_relax with smp_cond_load_relaxed

[Mihai Carabas <mihai.carabas@xxxxxxxxxx>]

La 29.01.2024 16:52, Rafael J. Wysocki a scris:
> On Mon, Jan 29, 2024 at 12:56 PM Mihai Carabas <mihai.carabas@xxxxxxxxxx> wrote:
> > cpu_relax on ARM64 does a simple "yield". Thus we replace it with
> > smp_cond_load_relaxed which basically does a "wfe".
> >
> > Suggested-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> > Signed-off-by: Mihai Carabas <mihai.carabas@xxxxxxxxxx>
> > ---
> >   drivers/cpuidle/poll_state.c | 14 +++++++++-----
> >   1 file changed, 9 insertions(+), 5 deletions(-)
> >
> > diff --git a/drivers/cpuidle/poll_state.c b/drivers/cpuidle/poll_state.c
> > index 9b6d90a72601..440cd713e39a 100644
> > --- a/drivers/cpuidle/poll_state.c
> > +++ b/drivers/cpuidle/poll_state.c
> > @@ -26,12 +26,16 @@ static int __cpuidle poll_idle(struct cpuidle_device *dev,
> >
> >                  limit = cpuidle_poll_time(drv, dev);
> >
> > -               while (!need_resched()) {
> > -                       cpu_relax();
> > -                       if (loop_count++ < POLL_IDLE_RELAX_COUNT)
> > -                               continue;
> > -
> > +               for (;;) {
> >                          loop_count = 0;
> > +
> > +                       smp_cond_load_relaxed(&current_thread_info()->flags,
> > +                                             (VAL & _TIF_NEED_RESCHED) ||
> > +                                             (loop_count++ >= POLL_IDLE_RELAX_COUNT));
> The inner parens are not necessary AFAICS.

Provides better reading. Do you want to remove these?

> Also, doesn't this return a value which can be used for checking if
> _TIF_NEED_RESCHED is set instead of the condition below?

Yes, indeed - should I modify this check? (somehow I wanted to preserve 
the original check)

Thank you,
Mihai

> > +
> > +                       if (loop_count < POLL_IDLE_RELAX_COUNT)
> > +                               break;
> > +
> >                          if (local_clock_noinstr() - time_start > limit) {
> >                                  dev->poll_time_limit = true;
> >                                  break;
> > --