Re: [PATCH v3 2/3] powerpc/perf: Update prev_count of event to get accurate values

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential
issue to consider:
- [Low] powerpc/perf: dead store to prev_data from local64_read() in
  vpa_pmu_read()
--

commit e6b6a01ca205d4ad136547505bcf1dcf984b82ba
Author: Gautam Menghani <gautam@linux.ibm.com>

powerpc/perf: Update prev_count of event to get accurate values

This commit updates event->hw.prev_count in vpa_pmu_read() using
local64_xchg() so that consecutive counter reads accumulate only the delta
since the last read.

> diff --git a/arch/powerpc/perf/vpa-pmu.c b/arch/powerpc/perf/vpa-pmu.c
> index 01d9196fcb0f6..3296877eb317b 100644
> --- a/arch/powerpc/perf/vpa-pmu.c
> +++ b/arch/powerpc/perf/vpa-pmu.c
> @@ -132,6 +132,7 @@ static void vpa_pmu_read(struct perf_event *event)
>  
>  	prev_data = local64_read(&event->hw.prev_count);
>  	new_data = get_counter_data(event);
> +	prev_data = local64_xchg(&event->hw.prev_count, new_data);

[Severity: Low]
Can the earlier local64_read(&event->hw.prev_count) call be dropped now
that local64_xchg() unconditionally overwrites prev_data before it is
used?  Because local64_read() expands to a volatile READ_ONCE() load, the
compiler cannot elide the redundant read on every vpa_pmu_read() call.

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20261106153136.330165-1-gautam@linux.ibm.com?part=2