Re: [PATCH v4 5/7] alpha/perf: implement ProfileMe sampling

[sashiko-bot@kernel.org]

> The 21264 (EV67 and later) and 21364 have two performance counting modes,
> and the driver only uses Aggregate mode. Its samples report the PC at
> which the counter interrupt was taken, which lands a variable number of
> instructions after the one that overflowed the counter. On an EV7 a
> 16-instruction user loop shows up at only 4 PCs, anywhere from 15
> instructions before to 12 after the real one.
> 
> In ProfileMe mode, the instruction that overflows PCTR0 opens a window
> during which PCTR1 counts, and when it closes PALcode delivers the
> interrupt with that instruction's PC latched in PMPC.
> 
> Select ProfileMe with the new profileme config bit or with precise_ip,
> since PMPC has no skid. PCTR_CTL[SL0] selects the mode for both
> counters, so a ProfileMe event is scheduled alone and perf core
> multiplexes it with other hardware events. The event type chooses the
> [ ... ]
> wrong path are sampled at their own PC, as ProfileMe defines them.
> 
> Suggested-by: Magnus Lindholm <linmag7@gmail.com>
> Assisted-by: Claude:claude-opus-5-5
> 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/20261102-alpha-profileme-v4-0-61e00f5e1da8@gmail.com?part=5