Re: [PATCH 1/2] perf/x86/amd/uncore: Remove redundant event slot scan

[sashiko-bot@kernel.org]

> amd_uncore_add() first checks the slot recorded in event->hw.idx. If
> that misses, it scans ctx->events[] for the event before looking for a
> free slot.
> 
> Perf serializes ->add() and ->del() for an event. Initialization sets
> idx to -1. A successful ->add() claims a slot and records its index
> before returning, while ->del() clears the slot before resetting idx.
> CPU context migration follows the same delete/add sequence.
> 
> Thus an installed event can only reside at the recorded index. If the
> direct check misses, the event is not present in ctx->events[].
> 
> Remove the redundant scan. This avoids walking all counters before each
> new slot search after multiplexing has scheduled an event out.
> 
> On a host running a production workload in the Meta fleet, amd_uncore_add()
> was called 56,575 times per second from mux rotation. Its 16-counter DF PMU
> makes each failed search scan 16 pointers across two cache lines.
> 
> Cc: Sandipan Das <sandipan.das@amd.com>
> Signed-off-by: Usama Arif <usama.arif@linux.dev>

Sashiko has reviewed this patch and found no issues. It looks great!

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Sashiko AI review · https://sashiko.dev/#/patchset/20260921153108.2713179-1-usama.arif@linux.dev?part=1