Re: [PATCH v6 05/13] alpha/perf: ignore overflow interrupts the counter does not account for

[sashiko-bot@kernel.org]

> The interrupt handler adds a full counter period to the count of the
> event on the counter that interrupted, which goes wrong in two cases.
> 
> Rewriting a counter does not cancel its pending interrupt: wrperfmon has
> no way to, and PALcode clears the request only when delivering it. An
> interrupt pending when an event is reloaded, or when perf moves another
> event onto the counter, is then taken as an overflow of a counter that
> has not overflowed. It adds a bogus period to the count and can emit a
> false sample.
> 
> And alpha_perf_event_update() already adds the period when a read finds
> that the counter overflowed before its interrupt was taken. The
> interrupt then adds it again.
> 
> An overflow leaves the counter just past zero, below the value it was
> written with, while a counter that has not overflowed is at or above it.
> Ignore an interrupt for a counter in the latter state, unless a read
> has already counted the overflow, in which case add nothing. For this to
> hold, write the counters at most half their range below overflow, as
> armpmu_event_set_period() does. Counting events then take an interrupt
> every 2^19 events rather than every 2^20.
> 
> Fixes: 979f867191f8 ("alpha: implement HW performance events on the EV67 and later CPUs")
> Suggested-by: Ian Rogers <irogers@google.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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