linux-kernel - [tip: perf/core] perf: Clean up mediated vPMU accounting

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Message-ID: <176597507837.510.17276279543690494180.tip-bot2@tip-bot2>
Date: Wed, 17 Dec 2025 12:37:58 -0000
From: "tip-bot2 for Peter Zijlstra" <tip-bot2@...utronix.de>
To: linux-tip-commits@...r.kernel.org
Cc: "Peter Zijlstra (Intel)" <peterz@...radead.org>, x86@...nel.org,
 linux-kernel@...r.kernel.org
Subject: [tip: perf/core] perf: Clean up mediated vPMU accounting

The following commit has been merged into the perf/core branch of tip:

Commit-ID:     3cb3c2f6886f9489df13de8efe7a1e803a3f21ea
Gitweb:        https://git.kernel.org/tip/3cb3c2f6886f9489df13de8efe7a1e803a3f21ea
Author:        Peter Zijlstra <peterz@...radead.org>
AuthorDate:    Wed, 17 Dec 2025 12:08:01 +01:00
Committer:     Peter Zijlstra <peterz@...radead.org>
CommitterDate: Wed, 17 Dec 2025 13:31:09 +01:00

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@...radead.org>
Link: https://patch.msgid.link/20251208115156.GE3707891@noisy.programming.kicks-ass.net
---
 kernel/events/core.c |  9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/kernel/events/core.c b/kernel/events/core.c
index dd842a4..e6a4b1e 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -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;