[PATCH 2/2] perf/x86/amd/uncore: Free counter slot by index

[Usama Arif]

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.

Cc: Sandipan Das <sandipan.das@xxxxxxx>
Signed-off-by: Usama Arif <usama.arif@xxxxxxxxx>
---
 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 53be8efaedc5d..39287640bf1ea 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;
 }
-- 
2.53.0-Meta