linux-kernel - Re: [PATCH] perf/core: Fix warning warning due to unordred pmu_ctx

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Message-ID: <a6a9a81a-3479-4d77-a71f-35f35d391482@huaweicloud.com>
Date: Tue, 21 Jan 2025 09:59:14 +0800
From: Luo Gengkun <luogengkun@...weicloud.com>
To: "Liang, Kan" <kan.liang@...ux.intel.com>, peterz@...radead.org
Cc: mingo@...hat.com, acme@...nel.org, namhyung@...nel.org,
 mark.rutland@....com, alexander.shishkin@...ux.intel.com, jolsa@...nel.org,
 irogers@...gle.com, adrian.hunter@...el.com, ravi.bangoria@....com,
 linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] perf/core: Fix warning warning due to unordred
 pmu_ctx_list


On 2025/1/21 4:49, Liang, Kan wrote:
> A redundant "warning" is in the title.
>
> On 2025-01-20 6:43 a.m., Luo Gengkun wrote:
>> 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 when inheritance is performed, it traverses the ordered
>> groups of events, and inserts the new perf_event_pmu_context into
>> child_ctx->pmu_ctx_list which is unordered. So the order of pmu_ctx_list in
>> the parent and child may be different.
> I think the order of pmu_ctx_list for the parent should be impacted by
> the time when an event/pmu is added.
> While the order for a child should be impacted by the event order in the
> pinned_groups and flexible_groups.

Yes, so the order of pmu_ctx_list for the parent and child maybe 
different because

of this point. I will make it clear in the commit message.

>> 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 open a lbr event, so it will alloc task_ctx_data, and
>> then open tracepoint and software events, so the parent ctx will have 3
>> different perf_event_pmu_contexts. When doing inherit, child ctx will
>> insert the perf_event_pmu_context in another order then the warning will
>> trigger.
>>
>> To fix this problem, add pmu_ctx_insertion_sort to make sure the
>> pmu_ctx_list is ordered.
>>
>> Fixes: bd2756811766 ("perf: Rewrite core context handling")
>> Signed-off-by: Luo Gengkun <luogengkun@...weicloud.com>
>> ---
>>   kernel/events/core.c | 22 ++++++++++++++++++++--
>>   1 file changed, 20 insertions(+), 2 deletions(-)
>>
>> diff --git a/kernel/events/core.c b/kernel/events/core.c
>> index 95b01a51139d..1bdff3ef0ce2 100644
>> --- a/kernel/events/core.c
>> +++ b/kernel/events/core.c
>> @@ -4953,6 +4953,24 @@ find_get_context(struct task_struct *task, struct perf_event *event)
>>   	return ERR_PTR(err);
>>   }
>>   
>> +/*
>> + * This function ensures that ctx->pmu_ctx_list is ordered, so that no warning
>> + * is triggered due to prev_epc->pmu != next_epc->pmu.
>> + */
>> +static void pmu_ctx_insertion_sort(struct perf_event_pmu_context *new,
>> +				   struct perf_event_context *ctx)
>> +{
>> +	struct perf_event_pmu_context *epc;
>> +
>> +	lockdep_assert_held(&ctx->lock);
>> +
>> +	list_for_each_entry(epc, &ctx->pmu_ctx_list, pmu_ctx_entry) {
>> +		if (epc->pmu > new->pmu)
>> +			break;
>> +	}
>> +	list_add(&new->pmu_ctx_entry, epc->pmu_ctx_entry.prev);
>> +}
>> +
>>   static struct perf_event_pmu_context *
>>   find_get_pmu_context(struct pmu *pmu, struct perf_event_context *ctx,
>>   		     struct perf_event *event)
>> @@ -4974,7 +4992,7 @@ find_get_pmu_context(struct pmu *pmu, struct perf_event_context *ctx,
>>   		if (!epc->ctx) {
>>   			atomic_set(&epc->refcount, 1);
>>   			epc->embedded = 1;
>> -			list_add(&epc->pmu_ctx_entry, &ctx->pmu_ctx_list);
>> +			pmu_ctx_insertion_sort(epc, ctx);
> The CPU event and per-task event should have a different ctx.
> The warning should only be triggered for the per-task event, right?
> If so, I don't think a sort is required here.
Yes, the ctx is extracted from task, so only sort the task ctx should 
fix this problem.
>
>>   			epc->ctx = ctx;
>>   		} else {
>>   			WARN_ON_ONCE(epc->ctx != ctx);
>> @@ -5021,7 +5039,7 @@ find_get_pmu_context(struct pmu *pmu, struct perf_event_context *ctx,
>>   	printk(KERN_INFO
>>   		"lgk: ctx %p insert pmu ctx %p, pmu is %p!\n", ctx, epc, epc->pmu);
> Seems your debug code. Please send a clean patch.
Sorry about this.
>>   
>> -	list_add(&epc->pmu_ctx_entry, &ctx->pmu_ctx_list);
>> +	pmu_ctx_insertion_sort(epc, ctx);
> I think the pmu_ctx_list has already traversed to find a matched pmu
> right before. The traverse in the pmu_ctx_insertion_sort() can be avoided.
>
> Thanks,
> Kan
Thanks for the review, I will send PATCH v2 later.
>>   	epc->ctx = ctx;
>>   
>>   found_epc: