From 6350de8671b94afb7691d110f63bcda42f658a69 Mon Sep 17 00:00:00 2001 From: Usama Arif Date: Mon, 21 Sep 2026 08:30:15 -0700 Subject: [PATCH] perf/x86/amd/uncore: Free counter slot by index amd_uncore_del() scans ctx->events[] from slot zero and attempts a compare-exchange until it finds the event. amd_uncore_add() records the slot it claimed in event->hw.idx. Perf calls ->del() only after a successful ->add(), and the driver never moves an installed event between slots. The index therefore remains valid until deletion. Use the recorded index directly and retain the compare-exchange ownership check. Warn if it fails, as that means the driver lost track of the slot. Deleting all events from a full PMU with N counters now requires N compare-exchanges instead of N * (N + 1) / 2. On a host running a production workload in the Meta fleet, amd_uncore_del() was called 56,575 times per second from mux rotation. Signed-off-by: Usama Arif Signed-off-by: Peter Zijlstra (Intel) Reviewed-by: Sandipan Das Link: https://patch.msgid.link/20260921153108.2713179-3-usama.arif@linux.dev --- arch/x86/events/amd/uncore.c | 10 +++------- 1 file changed, 3 insertions(+), 7 deletions(-) diff --git a/arch/x86/events/amd/uncore.c b/arch/x86/events/amd/uncore.c index c1158f3691d4..7aa5a5b652a2 100644 --- a/arch/x86/events/amd/uncore.c +++ b/arch/x86/events/amd/uncore.c @@ -246,19 +246,15 @@ static int amd_uncore_add(struct perf_event *event, int flags) static void amd_uncore_del(struct perf_event *event, int flags) { - int i; struct amd_uncore_pmu *pmu = event_to_amd_uncore_pmu(event); struct amd_uncore_ctx *ctx = *per_cpu_ptr(pmu->ctx, event->cpu); struct hw_perf_event *hwc = &event->hw; + struct perf_event *old = event; event->pmu->stop(event, PERF_EF_UPDATE); - for (i = 0; i < pmu->num_counters; i++) { - struct perf_event *tmp = event; - - if (try_cmpxchg(&ctx->events[i], &tmp, NULL)) - break; - } + /* ->del() follows a successful ->add(), so hwc->idx owns this slot. */ + WARN_ON_ONCE(!try_cmpxchg(&ctx->events[hwc->idx], &old, NULL)); hwc->idx = -1; }