Re: [PATCH v3] perf/core: Fix missing read event generation on task exit

[James Clark <james.clark@linaro.org>]

On 03/03/2026 2:29 pm, Peter Zijlstra wrote:
> 
> Sorry, things got lost again :-(
> 
> On Mon, Feb 09, 2026 at 04:12:39PM +0000, James Clark wrote:
> 
>>>>     @@ -41,10 +41,11 @@ static void perf_output_get_handle(struct
>>>>     perf_output_handle *handle)
>>>>     {
>>>>    	struct perf_buffer *rb = handle->rb;
>>>>
>>>>    	preempt_disable();
>>>>
>>>>     +	WARN_ON(handle->event->cpu != smp_processor_id());
>>>>
> 
>>> ---
>>> diff --git a/kernel/events/core.c b/kernel/events/core.c
>>> index 5b5cb620499e..f566ad55b4fb 100644
>>> --- a/kernel/events/core.c
>>> +++ b/kernel/events/core.c
>>> @@ -14086,7 +14086,7 @@ static void sync_child_event(struct perf_event *child_event,
>>>    	u64 child_val;
>>>    	if (child_event->attr.inherit_stat) {
>>> -		if (task && task != TASK_TOMBSTONE)
>>> +		if (task && task != TASK_TOMBSTONE && event_filter_match(child_event))
>>>    			perf_event_read_event(child_event, task);
>>>    	}
>>
>> Turns out I tested this before with "child_event->cpu ==
>> raw_smp_processor_id()" rather than using event_filter_match() so I missed
>> that the loop over all the events needs to be wrapped with
>> preempt_disable(). But that can't be done because perf_event_exit_event()
>> takes a mutex.
>>
>> I don't think the preempt_disable() can be on any smaller region than
>> outside the entire loop otherwise you can get rescheduled between
>> event_filter_match() checks and end up failing them all and not writing any
>> event out at all.
> 
> Ooh, cute. That's annoying. This is specific to using per-task-per-cpu
> buffers afaict, nevertheless those ought to work.
> 
> Also, I suspect your WARN would trigger before c418d8b4d7a4 as well, if
> it wouldn't have been for that TOMBSTONE thing.
> 
> How to fix this mess... perhaps something like this, except now I worry
> about the revoke and remove_on_exec cases; do they want something?
> 
> 

So the fixes here seem to fix the two problems for me, didn't see any 
issues in the Perf tests either. Not sure about remove_on_exec though, I 
didn't test that.

> ---
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index 03ced7aad309..d0aae76cabf4 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -4727,7 +4727,6 @@ static void perf_event_enable_on_exec(struct perf_event_context *ctx)
>   static void perf_remove_from_owner(struct perf_event *event);
>   static void perf_event_exit_event(struct perf_event *event,
>   				  struct perf_event_context *ctx,
> -				  struct task_struct *task,
>   				  bool revoke);
>   
>   /*
> @@ -4755,7 +4754,7 @@ static void perf_event_remove_on_exec(struct perf_event_context *ctx)
>   
>   		modified = true;
>   
> -		perf_event_exit_event(event, ctx, ctx->task, false);
> +		perf_event_exit_event(event, ctx, false);
>   	}
>   
>   	raw_spin_lock_irqsave(&ctx->lock, flags);
> @@ -12863,7 +12862,7 @@ static void __pmu_detach_event(struct pmu *pmu, struct perf_event *event,
>   	/*
>   	 * De-schedule the event and mark it REVOKED.
>   	 */
> -	perf_event_exit_event(event, ctx, ctx->task, true);
> +	perf_event_exit_event(event, ctx, true);
>   
>   	/*
>   	 * All _free_event() bits that rely on event->pmu:
> @@ -14424,17 +14423,11 @@ void perf_pmu_migrate_context(struct pmu *pmu, int src_cpu, int dst_cpu)
>   }
>   EXPORT_SYMBOL_GPL(perf_pmu_migrate_context);
>   
> -static void sync_child_event(struct perf_event *child_event,
> -			     struct task_struct *task)
> +static void sync_child_event(struct perf_event *child_event)
>   {
>   	struct perf_event *parent_event = child_event->parent;
>   	u64 child_val;
>   
> -	if (child_event->attr.inherit_stat) {
> -		if (task && task != TASK_TOMBSTONE)
> -			perf_event_read_event(child_event, task);
> -	}
> -
>   	child_val = perf_event_count(child_event, false);
>   
>   	/*
> @@ -14450,7 +14443,6 @@ static void sync_child_event(struct perf_event *child_event,
>   static void
>   perf_event_exit_event(struct perf_event *event,
>   		      struct perf_event_context *ctx,
> -		      struct task_struct *task,
>   		      bool revoke)
>   {
>   	struct perf_event *parent_event = event->parent;
> @@ -14476,7 +14468,7 @@ perf_event_exit_event(struct perf_event *event,
>   		attach_state = READ_ONCE(event->attach_state);
>   
>   		if (attach_state & PERF_ATTACH_CHILD)
> -			sync_child_event(event, task);
> +			sync_child_event(event);
>   	}
>   
>   	if (revoke)
> @@ -14517,7 +14509,7 @@ perf_event_exit_event(struct perf_event *event,
>   static void perf_event_exit_task_context(struct task_struct *task, bool exit)
>   {
>   	struct perf_event_context *ctx, *clone_ctx = NULL;
> -	struct perf_event *child_event, *next;
> +	struct perf_event *event, *next;
>   
>   	ctx = perf_pin_task_context(task);
>   	if (!ctx)
> @@ -14564,11 +14556,21 @@ static void perf_event_exit_task_context(struct task_struct *task, bool exit)
>   	 * won't get any samples after PERF_RECORD_EXIT. We can however still
>   	 * get a few PERF_RECORD_READ events.
>   	 */
> -	if (exit)
> +	if (exit) {
>   		perf_event_task(task, ctx, 0);
>   
> -	list_for_each_entry_safe(child_event, next, &ctx->event_list, event_entry)
> -		perf_event_exit_event(child_event, ctx, exit ? task : NULL, false);
> +		guard(raw_spinlock_irq)(&ctx->lock);
> +		list_for_each_entry(event, &ctx->event_list, event_entry) {
> +			if (event->attr.inherit_stat) {
> +				if (task && task != TASK_TOMBSTONE &&
> +				    event_filter_match(event))
> +					perf_event_read_event(event, task);
> +			}
> +		}
> +	}
> +
> +	list_for_each_entry_safe(event, next, &ctx->event_list, event_entry)
> +		perf_event_exit_event(event, ctx, false);
>   
>   	mutex_unlock(&ctx->mutex);
>   
> 
>> While debugging I also noticed another issue with these per-thread count
>> records. perf_event_exit_event() only does anything if the event has a
>> parent. But the context switch optimization means that sometimes threads
>> re-use the original event which has no parent. So randomly you get threads
>> that are missing from the output.
>>
>> There is a comment that mentions this under the parent check:
>>
>>    if (parent_event) {
>>      /*
>>       * Do not destroy the 'original' grouping; because of the
>>       * context switch optimization the original events could've
>>       * ended up in a random child task.
>>
>> But I'm not sure if that was supposed to be worked around some other way and
>> it's now broken, or it was a known limitation of the implementation from the
>> beginning? But right now it randomly misses one of the threads and includes
>> the main thread counts, or includes all the threads and doesn't include the
>> main thread counts if no context switch optimisation was done.
>>
>> The perf record docs don't say anything that you wouldn't expect all threads
>> to be there:
>>
>>      -s, --stat            per thread counts
> 
> Ha! So commit bfbd3381e63a ("perf_counter: Implement more accurate per
> task statistics") actually mentions that issue and tries to mitigate,
> but I think I indeed missed a case.
> 
> I suppose the easiest fix is this, but urgh...
> 
> ---
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index 03ced7aad309..c38acf1e2f43 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -3664,6 +3664,9 @@ static int context_equiv(struct perf_event_context *ctx1,
>   	if (ctx1->pin_count || ctx2->pin_count)
>   		return 0;
>   
> +	if (ctx1->nr_stat || ctx2->nr_stat)
> +		return 0;
> +
>   	/* If ctx1 is the parent of ctx2 */
>   	if (ctx1 == ctx2->parent_ctx && ctx1->generation == ctx2->parent_gen)
>   		return 1;
> @@ -3677,76 +3680,13 @@ static int context_equiv(struct perf_event_context *ctx1,
>   	 * hierarchy, see perf_event_init_context().
>   	 */
>   	if (ctx1->parent_ctx && ctx1->parent_ctx == ctx2->parent_ctx &&
> -			ctx1->parent_gen == ctx2->parent_gen)
> +	    ctx1->parent_gen == ctx2->parent_gen)
>   		return 1;
>   
>   	/* Unmatched */
>   	return 0;
>   }
>   
> -static void __perf_event_sync_stat(struct perf_event *event,
> -				     struct perf_event *next_event)
> -{
> -	u64 value;
> -
> -	if (!event->attr.inherit_stat)
> -		return;
> -
> -	/*
> -	 * Update the event value, we cannot use perf_event_read()
> -	 * because we're in the middle of a context switch and have IRQs
> -	 * disabled, which upsets smp_call_function_single(), however
> -	 * we know the event must be on the current CPU, therefore we
> -	 * don't need to use it.
> -	 */
> -	perf_pmu_read(event);
> -
> -	perf_event_update_time(event);
> -
> -	/*
> -	 * In order to keep per-task stats reliable we need to flip the event
> -	 * values when we flip the contexts.
> -	 */
> -	value = local64_read(&next_event->count);
> -	value = local64_xchg(&event->count, value);
> -	local64_set(&next_event->count, value);
> -
> -	swap(event->total_time_enabled, next_event->total_time_enabled);
> -	swap(event->total_time_running, next_event->total_time_running);
> -
> -	/*
> -	 * Since we swizzled the values, update the user visible data too.
> -	 */
> -	perf_event_update_userpage(event);
> -	perf_event_update_userpage(next_event);
> -}
> -
> -static void perf_event_sync_stat(struct perf_event_context *ctx,
> -				   struct perf_event_context *next_ctx)
> -{
> -	struct perf_event *event, *next_event;
> -
> -	if (!ctx->nr_stat)
> -		return;
> -
> -	update_context_time(ctx);
> -
> -	event = list_first_entry(&ctx->event_list,
> -				   struct perf_event, event_entry);
> -
> -	next_event = list_first_entry(&next_ctx->event_list,
> -					struct perf_event, event_entry);
> -
> -	while (&event->event_entry != &ctx->event_list &&
> -	       &next_event->event_entry != &next_ctx->event_list) {
> -
> -		__perf_event_sync_stat(event, next_event);
> -
> -		event = list_next_entry(event, event_entry);
> -		next_event = list_next_entry(next_event, event_entry);
> -	}
> -}
> -
>   static void perf_ctx_sched_task_cb(struct perf_event_context *ctx,
>   				   struct task_struct *task, bool sched_in)
>   {
> @@ -3835,8 +3775,6 @@ perf_event_context_sched_out(struct task_struct *task, struct task_struct *next)
>   			RCU_INIT_POINTER(next->perf_event_ctxp, ctx);
>   
>   			do_switch = 0;
> -
> -			perf_event_sync_stat(ctx, next_ctx);
>   		}
>   		raw_spin_unlock(&next_ctx->lock);
>   		raw_spin_unlock(&ctx->lock);