Re: [PATCH] drm/i915/pmu: Use local64_try_cmpxchg in i915_pmu_event_read

[Uros Bizjak]

On Tue, Jul 4, 2023 at 9:28 AM Jani Nikula <jani.nikula@xxxxxxxxxxxxxxx> wrote:
>
> On Mon, 03 Jul 2023, Uros Bizjak <ubizjak@xxxxxxxxx> wrote:
> > Use local64_try_cmpxchg instead of local64_cmpxchg (*ptr, old, new) == old
> > in i915_pmu_event_read.  x86 CMPXCHG instruction returns success in ZF flag,
> > so this change saves a compare after cmpxchg (and related move instruction
> > in front of cmpxchg).
> >
> > Also, try_cmpxchg implicitly assigns old *ptr value to "old" when cmpxchg
> > fails. There is no need to re-read the value in the loop.
> >
> > No functional change intended.
> >
> > Cc: Jani Nikula <jani.nikula@xxxxxxxxxxxxxxx>
> > Cc: Joonas Lahtinen <joonas.lahtinen@xxxxxxxxxxxxxxx>
> > Cc: Rodrigo Vivi <rodrigo.vivi@xxxxxxxxx>
> > Cc: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxxxxxxxx>
> > Cc: David Airlie <airlied@xxxxxxxxx>
> > Cc: Daniel Vetter <daniel@xxxxxxxx>
> > Signed-off-by: Uros Bizjak <ubizjak@xxxxxxxxx>
> > ---
> >  drivers/gpu/drm/i915/i915_pmu.c | 9 ++++-----
> >  1 file changed, 4 insertions(+), 5 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/i915/i915_pmu.c b/drivers/gpu/drm/i915/i915_pmu.c
> > index d35973b41186..108b675088ba 100644
> > --- a/drivers/gpu/drm/i915/i915_pmu.c
> > +++ b/drivers/gpu/drm/i915/i915_pmu.c
> > @@ -696,12 +696,11 @@ static void i915_pmu_event_read(struct perf_event *event)
> >               event->hw.state = PERF_HES_STOPPED;
> >               return;
> >       }
> > -again:
> > -     prev = local64_read(&hwc->prev_count);
> > -     new = __i915_pmu_event_read(event);
> >
> > -     if (local64_cmpxchg(&hwc->prev_count, prev, new) != prev)
> > -             goto again;
> > +     prev = local64_read(&hwc->prev_count);
> > +     do {
> > +             new = __i915_pmu_event_read(event);
> > +     } while (!local64_try_cmpxchg(&hwc->prev_count, &prev, new));
>
> You could save everyone a lot of time by actually documenting what these
> functions do. Assume you don't know what local64_try_cmpxchg() does, and
> see how many calls you have to go through to figure it out.

These functions are documented in Documentation/atomic_t.txt (under
"RMW ops:" section), and the difference is explained in a separate
section "CMPXCHG vs TRY_CMPXCGS" in the same file.

Uros.

> Because the next time I encounter this code or a patch like this, I'm
> probably going to have to do that again.
>
> To me, the old one was more readable. The optimization is meaningless to
> me if it's not quantified but reduces readability.
>
>
> BR,
> Jani.
>
>
> >
> >       local64_add(new - prev, &event->count);
> >  }
>
> --
> Jani Nikula, Intel Open Source Graphics Center