[PATCH 1/2] arm_pmu: fix event CPU filtering

From: Mark Rutland
Date: Thu Feb 16 2023 - 09:13:02 EST


Janne reports that perf has been broken on Apple M1 as of commit:

bd27568117664b8b ("perf: Rewrite core context handling")

That commit replaced the pmu::filter_match() callback with
pmu::filter(), whose return value has the opposite polarity, with true
implying events should be ignored rather than scheduled. While an
attempt was made to update the logic in armv8pmu_filter() and
armpmu_filter() accordingly, the return value remains inverted in a
couple of cases:

* If the arm_pmu does not have an arm_pmu::filter() callback,
armpmu_filter() will always return whether the CPU is supported rather
than whether the CPU is not supported.

As a result, the perf core will not schedule events on supported CPUs,
resulting in a loss of events. Additionally, the perf core will
attempt to schedule events on unsupported CPUs, but this will be
rejected by armpmu_add(), which may result in a loss of events from
other PMUs on those unsupported CPUs.

* If the arm_pmu does have an arm_pmu::filter() callback, and
armpmu_filter() is called on a CPU which is not supported by the
arm_pmu, armpmu_filter() will return false rather than true.

As a result, the perf core will attempt to schedule events on
unsupported CPUs, but this will be rejected by armpmu_add(), which may
result in a loss of events from other PMUs on those unsupported CPUs.

This means a loss of events can be seen with any arm_pmu driver, but
with the ARMv8 PMUv3 driver (which is the only arm_pmu driver with an
arm_pmu::filter() callback) the event loss will be more limited and may
go unnoticed, which is how this issue evaded testing so far.

Fix the CPU filtering by performing this consistently in
armpmu_filter(), and remove the redundant arm_pmu::filter() callback and
armv8pmu_filter() implementation.

Commit bd2756811766 also silently removed the CHAIN event filtering from
armv8pmu_filter(), which will be addressed by a separate patch without
using the filter callback.

Fixes: bd27568117664b8b ("perf: Rewrite core context handling")
Reported-by: Janne Grunau <j@xxxxxxxxxx>
Link: https://lore.kernel.org/asahi/20230215-arm_pmu_m1_regression-v1-1-f5a266577c8d@xxxxxxxxxx/
Signed-off-by: Mark Rutland <mark.rutland@xxxxxxx>
Cc: Will Deacon <will@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Ravi Bangoria <ravi.bangoria@xxxxxxx>
Cc: Asahi Lina <lina@xxxxxxxxxxxxx>
Cc: Eric Curtin <ecurtin@xxxxxxxxxx>
---
arch/arm64/kernel/perf_event.c | 7 -------
drivers/perf/arm_pmu.c | 8 +-------
include/linux/perf/arm_pmu.h | 1 -
3 files changed, 1 insertion(+), 15 deletions(-)

diff --git a/arch/arm64/kernel/perf_event.c b/arch/arm64/kernel/perf_event.c
index a5193f2146a6..3e43538f6b72 100644
--- a/arch/arm64/kernel/perf_event.c
+++ b/arch/arm64/kernel/perf_event.c
@@ -1023,12 +1023,6 @@ static int armv8pmu_set_event_filter(struct hw_perf_event *event,
return 0;
}

-static bool armv8pmu_filter(struct pmu *pmu, int cpu)
-{
- struct arm_pmu *armpmu = to_arm_pmu(pmu);
- return !cpumask_test_cpu(smp_processor_id(), &armpmu->supported_cpus);
-}
-
static void armv8pmu_reset(void *info)
{
struct arm_pmu *cpu_pmu = (struct arm_pmu *)info;
@@ -1258,7 +1252,6 @@ static int armv8_pmu_init(struct arm_pmu *cpu_pmu, char *name,
cpu_pmu->stop = armv8pmu_stop;
cpu_pmu->reset = armv8pmu_reset;
cpu_pmu->set_event_filter = armv8pmu_set_event_filter;
- cpu_pmu->filter = armv8pmu_filter;

cpu_pmu->pmu.event_idx = armv8pmu_user_event_idx;

diff --git a/drivers/perf/arm_pmu.c b/drivers/perf/arm_pmu.c
index 9b593f985805..40f70f83daba 100644
--- a/drivers/perf/arm_pmu.c
+++ b/drivers/perf/arm_pmu.c
@@ -550,13 +550,7 @@ static void armpmu_disable(struct pmu *pmu)
static bool armpmu_filter(struct pmu *pmu, int cpu)
{
struct arm_pmu *armpmu = to_arm_pmu(pmu);
- bool ret;
-
- ret = cpumask_test_cpu(cpu, &armpmu->supported_cpus);
- if (ret && armpmu->filter)
- return armpmu->filter(pmu, cpu);
-
- return ret;
+ return !cpumask_test_cpu(cpu, &armpmu->supported_cpus);
}

static ssize_t cpus_show(struct device *dev,
diff --git a/include/linux/perf/arm_pmu.h b/include/linux/perf/arm_pmu.h
index ef914a600087..525b5d64e394 100644
--- a/include/linux/perf/arm_pmu.h
+++ b/include/linux/perf/arm_pmu.h
@@ -100,7 +100,6 @@ struct arm_pmu {
void (*stop)(struct arm_pmu *);
void (*reset)(void *);
int (*map_event)(struct perf_event *event);
- bool (*filter)(struct pmu *pmu, int cpu);
int num_events;
bool secure_access; /* 32-bit ARM only */
#define ARMV8_PMUV3_MAX_COMMON_EVENTS 0x40
--
2.30.2