linux-kernel - RE: [PATCH v4 7/8] cpuidle/poll_state: replace cpu_relax with smp_cond_load

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Date: Tue, 27 Feb 2024 20:36:45 -0800
From: Ankur Arora <ankur.a.arora@...cle.com>
To: misono.tomohiro@...itsu.com
Cc: akpm@...ux-foundation.org, ankur.a.arora@...cle.com, arnd@...db.de,
        bp@...en8.de, catalin.marinas@....com, daniel.lezcano@...aro.org,
        dianders@...omium.org, hpa@...or.com, joao.m.martins@...cle.com,
        juerg.haefliger@...onical.com, kvm@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-pm@...r.kernel.org, mic@...ikod.net, mihai.carabas@...cle.com,
        mingo@...hat.com, npiggin@...il.com, pbonzini@...hat.com,
        peterz@...radead.org, pmladek@...e.com, rafael@...nel.org,
        rick.p.edgecombe@...el.com, tglx@...utronix.de, vkuznets@...hat.com,
        wanpengli@...cent.com, will@...nel.org, x86@...nel.org
Subject: RE: [PATCH v4 7/8] cpuidle/poll_state: replace cpu_relax with smp_cond_load_relaxed

Tomohiro Misono (Fujitsu) <misono.tomohiro@...itsu.com> writes:

> Hi,
> > Subject: [PATCH v4 7/8] cpuidle/poll_state: replace cpu_relax with smp_cond_load_relaxed
> > 
> > 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@...radead.org>
> > Signed-off-by: Mihai Carabas <mihai.carabas@...cle.com>
> > ---
> >  drivers/cpuidle/poll_state.c | 15 ++++++++++-----
> >  1 file changed, 10 insertions(+), 5 deletions(-)
> >
> > diff --git a/drivers/cpuidle/poll_state.c b/drivers/cpuidle/poll_state.c
> > index 9b6d90a72601..1e45be906e72 100644
> > --- a/drivers/cpuidle/poll_state.c
> > +++ b/drivers/cpuidle/poll_state.c
> > @@ -13,6 +13,7 @@
> >  static int __cpuidle poll_idle(struct cpuidle_device *dev,
> >                              struct cpuidle_driver *drv, int index)
> >  {
> > +     unsigned long ret;
> >       u64 time_start;
> >
> >       time_start = local_clock_noinstr();
> > @@ -26,12 +27,16 @@ 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;
> > +
> > +                     ret = smp_cond_load_relaxed(&current_thread_info()->flags,
> > +                                                 VAL & _TIF_NEED_RESCHED ||
> > +                                                 loop_count++ >= POLL_IDLE_RELAX_COUNT);
> > +
> > +                     if (!(ret & _TIF_NEED_RESCHED))
> > +                             break;
> 
> Should this be "if (ret & _TIF_NEED_RESCHED) since we want to break here
> if the flag is set, or am I misunderstood?

Yeah, you are right. The check is inverted.

I'll be re-spinning this series. Will fix. Though, it probably makes sense
to just keep the original "while (!need_resched())" check.

Thanks for the review.

--
ankur