Re: [PATCH 0/2] perf/x86/amd/uncore: Drop redundant counter slot searches

[Sandipan Das]

On 21-09-2026 21:00, Usama Arif wrote:
> Both amd_uncore_add() and amd_uncore_del() search ctx->events[] for an
> event whose counter slot is already recorded in event->hw.idx.
> 
> Perf serializes ->add() and ->del() for an event, and the driver never
> moves an installed event between slots, so an installed event can only
> be at the recorded index. Both searches are redundant.
> 
> Neither callback is normally hot. That changes once more events target
> a PMU than it has counters. Perf then multiplexes them: every mux tick
> deschedules the resident set and schedules the next one, so both callbacks
> run for each rotated event on every tick.
> 
> Meta's fleet-wide profiles attribute 1.1% of kernel CPU time, excluding
> do_idle(), to amd_uncore_add() and amd_uncore_del() combined. On the
> host that exposed this, a workload holding 15,782 perf event file
> descriptors drove 56,575 calls per second into each callback from mux
> rotation, against a 16-counter DF PMU.
> 
> Patch 1 removes the scan in amd_uncore_add(). It walks every counter
> before the free slot search, which is the common case once multiplexing
> has scheduled the event out.
> 
> Patch 2 makes amd_uncore_del() free the recorded slot directly instead
> of rescanning from slot zero. Deleting all events from a full PMU with
> N counters drops from N * (N + 1) / 2 compare-exchanges to N.
> 
> No functional change intended.
> 
> Usama Arif (2):
>   perf/x86/amd/uncore: Remove redundant event slot scan
>   perf/x86/amd/uncore: Free counter slot by index
> 
>  arch/x86/events/amd/uncore.c | 24 +++++++++---------------
>  1 file changed, 9 insertions(+), 15 deletions(-)
> 

For the series

Reviewed-by: Sandipan Das <sandipan.das@xxxxxxx>