linux-kernel - [PATCH 2/2] perf/core: Allow reading package events from perf_event_read

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Message-Id: <20230823055653.2964237-3-tero.kristo@linux.intel.com>
Date:   Wed, 23 Aug 2023 08:56:53 +0300
From:   Tero Kristo <tero.kristo@...ux.intel.com>
To:     dave.hansen@...ux.intel.com, tglx@...utronix.de, x86@...nel.org,
        bp@...en8.de
Cc:     artem.bityutskiy@...ux.intel.com, acme@...nel.org,
        bpf@...r.kernel.org, namhyung@...nel.org, mingo@...hat.com,
        linux-kernel@...r.kernel.org, linux-perf-users@...r.kernel.org,
        irogers@...gle.com, hpa@...or.com, mark.rutland@....com,
        jolsa@...nel.org, adrian.hunter@...el.com,
        alexander.shishkin@...ux.intel.com, peterz@...radead.org
Subject: [PATCH 2/2] perf/core: Allow reading package events from perf_event_read_local

Per-package perf events are typically registered with a single CPU only,
however they can be read across all the CPUs within the package.
Currently perf_event_read maps the event CPU according to the topology
information to avoid an unnecessary SMP call, however
perf_event_read_local deals with hard values and rejects a read with a
failure if the CPU is not the one exactly registered. Allow similar
mapping within the perf_event_read_local if the perf event in question
can support this.

This allows users like BPF code to read the package perf events properly
across different CPUs within a package.

Signed-off-by: Tero Kristo <tero.kristo@...ux.intel.com>
---
 kernel/events/core.c | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/kernel/events/core.c b/kernel/events/core.c
index 78ae7b6f90fd..37db7c003b79 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -4528,6 +4528,7 @@ int perf_event_read_local(struct perf_event *event, u64 *value,
 {
 	unsigned long flags;
 	int ret = 0;
+	int event_cpu;
 
 	/*
 	 * Disabling interrupts avoids all counter scheduling (context
@@ -4551,15 +4552,19 @@ int perf_event_read_local(struct perf_event *event, u64 *value,
 		goto out;
 	}
 
+	/* Allow reading a per-package perf-event from local CPU also */
+	event_cpu = READ_ONCE(event->oncpu);
+	event_cpu = __perf_event_read_cpu(event, event_cpu);
+
 	/* If this is a per-CPU event, it must be for this CPU */
 	if (!(event->attach_state & PERF_ATTACH_TASK) &&
-	    event->cpu != smp_processor_id()) {
+	    event_cpu != smp_processor_id()) {
 		ret = -EINVAL;
 		goto out;
 	}
 
 	/* If this is a pinned event it must be running on this CPU */
-	if (event->attr.pinned && event->oncpu != smp_processor_id()) {
+	if (event->attr.pinned && event_cpu != smp_processor_id()) {
 		ret = -EBUSY;
 		goto out;
 	}
@@ -4569,7 +4574,7 @@ int perf_event_read_local(struct perf_event *event, u64 *value,
 	 * or local to this CPU. Furthermore it means its ACTIVE (otherwise
 	 * oncpu == -1).
 	 */
-	if (event->oncpu == smp_processor_id())
+	if (event_cpu == smp_processor_id())
 		event->pmu->read(event);
 
 	*value = local64_read(&event->count);
-- 
2.25.1