[PATCH 6.6 071/254] LoongArch: Fix PMU counter allocation for mixed-type event groups

Greg Kroah-Hartman
2026-01-28T15:31:53Z

From: Lisa Robinson <lisa@bytefly.space>

commit a91f86e27087f250a5d9c89bb4a427b9c30fd815 upstream.

When validating a perf event group, validate_group() unconditionally
attempts to allocate hardware PMU counters for the leader, sibling
events and the new event being added.

This is incorrect for mixed-type groups. If a PERF_TYPE_SOFTWARE event
is part of the group, the current code still tries to allocate a hardware
PMU counter for it, which can wrongly consume hardware PMU resources and
cause spurious allocation failures.

Fix this by only allocating PMU counters for hardware events during group
validation, and skipping software events.

A trimmed down reproducer is as simple as this:

#include <stdio.h>
#include <assert.h>
#include <unistd.h>
#include <string.h>
#include <sys/syscall.h>
#include <linux/perf_event.h>

int main (int argc, char *argv[])
{
struct perf_event_attr attr = { 0 };
int fds[5];

attr.disabled = 1;
attr.exclude_kernel = 1;
attr.exclude_hv = 1;
attr.read_format = PERF_FORMAT_TOTAL_TIME_ENABLED |
PERF_FORMAT_TOTAL_TIME_RUNNING | PERF_FORMAT_ID | PERF_FORMAT_GROUP;
attr.size = sizeof (attr);

attr.type = PERF_TYPE_SOFTWARE;
attr.config = PERF_COUNT_SW_DUMMY;
fds[0] = syscall (SYS_perf_event_open, &attr, 0, -1, -1, 0);
assert (fds[0] >= 0);

attr.type = PERF_TYPE_HARDWARE;
attr.config = PERF_COUNT_HW_CPU_CYCLES;
fds[1] = syscall (SYS_perf_event_open, &attr, 0, -1, fds[0], 0);
assert (fds[1] >= 0);

attr.type = PERF_TYPE_HARDWARE;
attr.config = PERF_COUNT_HW_INSTRUCTIONS;
fds[2] = syscall (SYS_perf_event_open, &attr, 0, -1, fds[0], 0);
assert (fds[2] >= 0);

attr.type = PERF_TYPE_HARDWARE;
attr.config = PERF_COUNT_HW_BRANCH_MISSES;
fds[3] = syscall (SYS_perf_event_open, &attr, 0, -1, fds[0], 0);
assert (fds[3] >= 0);

attr.type = PERF_TYPE_HARDWARE;
attr.config = PERF_COUNT_HW_CACHE_REFERENCES;
fds[4] = syscall (SYS_perf_event_open, &attr, 0, -1, fds[0], 0);
assert (fds[4] >= 0);

printf ("PASSED\n");

return 0;
}

Cc: stable@vger.kernel.org
Fixes: b37042b2bb7c ("LoongArch: Add perf events support")
Signed-off-by: Lisa Robinson <lisa@bytefly.space>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
arch/loongarch/kernel/perf_event.c | 21 ++++++++++++++++++---
1 file changed, 18 insertions(+), 3 deletions(-)

--- a/arch/loongarch/kernel/perf_event.c
+++ b/arch/loongarch/kernel/perf_event.c
@@ -637,6 +637,18 @@ static const struct loongarch_perf_event
return pev;
}

+static inline bool loongarch_pmu_event_requires_counter(const struct perf_event *event)
+{
+ switch (event->attr.type) {
+ case PERF_TYPE_HARDWARE:
+ case PERF_TYPE_HW_CACHE:
+ case PERF_TYPE_RAW:
+ return true;
+ default:
+ return false;
+ }
+}
+
static int validate_group(struct perf_event *event)
{
struct cpu_hw_events fake_cpuc;
@@ -644,15 +656,18 @@ static int validate_group(struct perf_ev

memset(&fake_cpuc, 0, sizeof(fake_cpuc));

- if (loongarch_pmu_alloc_counter(&fake_cpuc, &leader->hw) < 0)
+ if (loongarch_pmu_event_requires_counter(leader) &&
+ loongarch_pmu_alloc_counter(&fake_cpuc, &leader->hw) < 0)
return -EINVAL;

for_each_sibling_event(sibling, leader) {
- if (loongarch_pmu_alloc_counter(&fake_cpuc, &sibling->hw) < 0)
+ if (loongarch_pmu_event_requires_counter(sibling) &&
+ loongarch_pmu_alloc_counter(&fake_cpuc, &sibling->hw) < 0)
return -EINVAL;
}

- if (loongarch_pmu_alloc_counter(&fake_cpuc, &event->hw) < 0)
+ if (loongarch_pmu_event_requires_counter(event) &&
+ loongarch_pmu_alloc_counter(&fake_cpuc, &event->hw) < 0)
return -EINVAL;

return 0;