Commit 3cb3c2f6 authored Dec 17, 2025 by Peter Zijlstra

perf: Clean up mediated vPMU accounting



The mediated_pmu_account_event() and perf_create_mediated_pmu()
functions implement the exclusion between '!exclude_guest' counters
and mediated vPMUs. Their implementation is basically identical,
except mirrored in what they count/check.

Make sure the actual implementations reflect this similarity.

Notably:
 - while perf_release_mediated_pmu() has an underflow check;
   mediated_pmu_unaccount_event() did not.
 - while perf_create_mediated_pmu() has an inc_not_zero() path;
   mediated_pmu_account_event() did not.

Also, the inc_not_zero() path can be outsite of
perf_mediated_pmu_mutex. The mutex must guard the 0->1 (of either
nr_include_guest_events or nr_mediated_pmu_vms) transition, but once a
counter is already non-zero, it can safely be incremented further.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://patch.msgid.link/20251208115156.GE3707891@noisy.programming.kicks-ass.net

parent 3006911f

kernel/events/core.c

+7 −2

Original line number	Diff line number	Diff line
		@@ -6344,8 +6344,10 @@ static int mediated_pmu_account_event(struct perf_event *event)
		if (!is_include_guest_event(event))
		return 0;

		guard(mutex)(&perf_mediated_pmu_mutex);
		if (atomic_inc_not_zero(&nr_include_guest_events))
		return 0;

		guard(mutex)(&perf_mediated_pmu_mutex);
		if (atomic_read(&nr_mediated_pmu_vms))
		return -EOPNOTSUPP;

		@@ -6358,6 +6360,9 @@ static void mediated_pmu_unaccount_event(struct perf_event *event)
		if (!is_include_guest_event(event))
		return;

		if (WARN_ON_ONCE(!atomic_read(&nr_include_guest_events)))
		return;

		atomic_dec(&nr_include_guest_events);
		}

		@@ -6373,10 +6378,10 @@ static void mediated_pmu_unaccount_event(struct perf_event *event)
		*/
		int perf_create_mediated_pmu(void)
		{
		guard(mutex)(&perf_mediated_pmu_mutex);
		if (atomic_inc_not_zero(&nr_mediated_pmu_vms))
		return 0;

		guard(mutex)(&perf_mediated_pmu_mutex);
		if (atomic_read(&nr_include_guest_events))
		return -EBUSY;