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

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: bd2756811766 ("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

diff --git a/kernel/events/core.c b/kernel/events/core.c
index 7dabbcaf825a08872fd4c4742b601c8545e91595..086d46d0969630cd73e375810e187541fb7d33e0 100644 (file)
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -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: