github-mirror/linux
1
0
Fork 0
mirror of https://github.com/torvalds/linux.git synced 2026-10-23 23:51:02 +02:00
Code Issues Actions Packages Projects Releases Wiki Activity Graph

perf/x86/amd/uncore: Free counter slot by index

Browse Source 
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 <usama.arif@linux.dev>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Sandipan Das <sandipan.das@amd.com>
Link: https://patch.msgid.link/20260921153108.2713179-3-usama.arif@linux.dev
This commit is contained in:
Usama Arif 2026-09-21 08:30:15 -07:00 committed by Peter Zijlstra
parent 4a8557a4e5
commit 6350de8671
1 changed files with 3 additions and 7 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

10
arch/x86/events/amd/uncore.c
View File

@ -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;
}