Re: [PATCH v2 5/5] alpha/perf: optionally randomize the ProfileMe sampling interval

[sashiko-bot@kernel.org]

> A fixed sampling interval can phase-lock with a loop whose length divides
> it, so that the same few instructions are profiled over and over while
> the rest of the loop is never seen. DCPI avoided this by randomizing the
> interval.
> 
> Add a randomize config bit that varies each interval by up to 1/16 of
> the period in either direction. Only ProfileMe events accept it, since
> Aggregate mode samples already land a variable distance from the
> instruction that overflowed. Counts are unaffected, because prev_count
> already records the value actually written to the counter. The jitter
> is computed with mul_u64_u32_shr() and the sum saturates, so that a
> period beyond 32 bits cannot overflow.
> 
> The jitter comes from a per-CPU prandom state rather than
> get_random_u32_below(), which takes the batched entropy local lock and
> triggers an invalid wait context report from lockdep in the PMI.
> 
> Signed-off-by: Matt Turner <mattst88@gmail.com>

Sashiko has reviewed this patch and found no issues.

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Sashiko AI review · https://sashiko.dev/#/patchset/20261030-alpha-profileme-v2-0-c702f1c65272@gmail.com?part=5