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perf/core: Order the PMU list to fix warning about unordered pmu_ctx_list

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Syskaller triggers a warning due to prev_epc->pmu != next_epc->pmu in
perf_event_swap_task_ctx_data(). vmcore shows that two lists have the same
perf_event_pmu_context, but not in the same order.

The problem is that the order of pmu_ctx_list for the parent is impacted by
the time when an event/PMU is added. While the order for a child is
impacted by the event order in the pinned_groups and flexible_groups. So
the order of pmu_ctx_list in the parent and child may be different.

To fix this problem, insert the perf_event_pmu_context to its proper place
after iteration of the pmu_ctx_list.

The follow testcase can trigger above warning:

 # perf record -e cycles --call-graph lbr -- taskset -c 3 ./a.out &
 # perf stat -e cpu-clock,cs -p xxx // xxx is the pid of a.out

 test.c

 void main() {
        int count = 0;
        pid_t pid;

        printf("%d running\n", getpid());
        sleep(30);
        printf("running\n");

        pid = fork();
        if (pid == -1) {
                printf("fork error\n");
                return;
        }
        if (pid == 0) {
                while (1) {
                        count++;
                }
        } else {
                while (1) {
                        count++;
                }
        }
 }

The testcase first opens an LBR event, so it will allocate task_ctx_data,
and then open tracepoint and software events, so the parent context will
have 3 different perf_event_pmu_contexts. On inheritance, child ctx will
insert the perf_event_pmu_context in another order and the warning will
trigger.

[ mingo: Tidied up the changelog. ]

Fixes: bd27568117 ("perf: Rewrite core context handling")
Signed-off-by: Luo Gengkun <luogengkun@huaweicloud.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Kan Liang <kan.liang@linux.intel.com>
Link: https://lore.kernel.org/r/20250122073356.1824736-1-luogengkun@huaweicloud.com
This commit is contained in:
Luo Gengkun 2025-01-22 07:33:56 +00:00 • committed by Ingo Molnar
parent 0fe8813baf
commit 2016066c66
1 changed files with 9 additions and 2 deletions
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11
kernel/events/core.c
View file

@ -4950,7 +4950,7 @@ static struct perf_event_pmu_context *
find_get_pmu_context(struct pmu *pmu, struct perf_event_context *ctx,
struct perf_event *event)
{
struct perf_event_pmu_context *new = NULL, *epc;
struct perf_event_pmu_context *new = NULL, *pos = NULL, *epc;
void *task_ctx_data = NULL;
if (!ctx->task) {
@ -5007,12 +5007,19 @@ find_get_pmu_context(struct pmu *pmu, struct perf_event_context *ctx,
atomic_inc(&epc->refcount);
goto found_epc;
}
/* Make sure the pmu_ctx_list is sorted by PMU type: */
if (!pos && epc->pmu->type > pmu->type)
pos = epc;
}
epc = new;
new = NULL;
list_add(&epc->pmu_ctx_entry, &ctx->pmu_ctx_list);
if (!pos)
list_add_tail(&epc->pmu_ctx_entry, &ctx->pmu_ctx_list);
else
list_add(&epc->pmu_ctx_entry, pos->pmu_ctx_entry.prev);
epc->ctx = ctx;
found_epc: