On Wed, 2023-07-05 at 07:53 -0700, Dave Jiang wrote:
>
>
> On 7/3/23 07:52, Uros Bizjak wrote:
> > Use local64_try_cmpxchg instead of local64_cmpxchg (*ptr, old, new) == old
> > in perfmon_pmu_event_update. 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: Fenghua Yu <fenghua.yu@xxxxxxxxx>
> > Cc: Dave Jiang <dave.jiang@xxxxxxxxx>
> > Cc: Vinod Koul <vkoul@xxxxxxxxxx>
> > Signed-off-by: Uros Bizjak <ubizjak@xxxxxxxxx>
>
>
> Cc: Tom Zanussi
>
> Tom do you mind review this patch? Thanks!
Looks fine to me.
Thanks,
Reviewed-by: Tom Zanussi <tom.zanussi@xxxxxxxxxxxxxxx>
>
> > ---
> > drivers/dma/idxd/perfmon.c | 7 +++----
> > 1 file changed, 3 insertions(+), 4 deletions(-)
> >
> > diff --git a/drivers/dma/idxd/perfmon.c b/drivers/dma/idxd/perfmon.c
> > index d73004f47cf4..fdda6d604262 100644
> > --- a/drivers/dma/idxd/perfmon.c
> > +++ b/drivers/dma/idxd/perfmon.c
> > @@ -245,12 +245,11 @@ static void perfmon_pmu_event_update(struct perf_event *event)
> > int shift = 64 - idxd->idxd_pmu->counter_width;
> > struct hw_perf_event *hwc = &event->hw;
> >
> > + prev_raw_count = local64_read(&hwc->prev_count);
> > do {
> > - prev_raw_count = local64_read(&hwc->prev_count);
> > new_raw_count = perfmon_pmu_read_counter(event);
> > - } while (local64_cmpxchg(&hwc->prev_count, prev_raw_count,
> > - new_raw_count) != prev_raw_count);
> > -
> > + } while (!local64_try_cmpxchg(&hwc->prev_count,
> > + &prev_raw_count, new_raw_count));
> > n = (new_raw_count << shift);
> > p = (prev_raw_count << shift);
> >
