Re: [RFC PATCH 1/1] perf/core: Wake up parent event if inherited event has no ring buffer

[Peter Zijlstra <peterz@infradead.org>]

On Mon, Dec 06, 2021 at 11:38:40AM +0000, James Clark wrote:
> When using per-process mode and event inheritance is set to true, forked
> processes will create a new perf events via inherit_event() ->
> perf_event_alloc(). But these events will not have ring buffers assigned
> to them. Any call to wakeup will be dropped if it's called on an event
> with no ring buffer assigned because that's the object that holds the
> wakeup list.
> 
> If the child event is disabled due to a call to perf_aux_output_begin()
> or perf_aux_output_end(), the wakeup is dropped leaving userspace
> hanging forever on the poll.
> 
> Normally the event is explicitly re-enabled by userspace after it wakes
> up to read the aux data, but in this case it does not get woken up so
> the event remains disabled.
> 
> This can be reproduced when using Arm SPE and 'stress' which forks once
> before running the workload. By looking at the list of aux buffers read,
> it's apparent that they stop after the fork:
> 
>   perf record -e arm_spe// -vvv -- stress -c 1
> 
> With this patch applied they continue to be printed. This behaviour
> doesn't happen when using systemwide or per-cpu mode.
> 
> Reported-by: Ruben Ayrapetyan <Ruben.Ayrapetyan@arm.com>
> Signed-off-by: James Clark <james.clark@arm.com>
> ---

Would this be the better patch?


---
diff --git a/kernel/events/core.c b/kernel/events/core.c
index 479c9e672ec4..b1c1928c0e7c 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -5985,6 +5985,8 @@ static void ring_buffer_attach(struct perf_event *event,
 	struct perf_buffer *old_rb = NULL;
 	unsigned long flags;
 
+	WARN_ON_ONCE(event->parent);
+
 	if (event->rb) {
 		/*
 		 * Should be impossible, we set this when removing
@@ -6042,6 +6044,9 @@ static void ring_buffer_wakeup(struct perf_event *event)
 {
 	struct perf_buffer *rb;
 
+	if (event->parent)
+		event = event->parent;
+
 	rcu_read_lock();
 	rb = rcu_dereference(event->rb);
 	if (rb) {
@@ -6055,6 +6060,9 @@ struct perf_buffer *ring_buffer_get(struct perf_event *event)
 {
 	struct perf_buffer *rb;
 
+	if (event->parent)
+		event = event->parent;
+
 	rcu_read_lock();
 	rb = rcu_dereference(event->rb);
 	if (rb) {
@@ -6763,7 +6771,7 @@ static unsigned long perf_prepare_sample_aux(struct perf_event *event,
 	if (WARN_ON_ONCE(READ_ONCE(sampler->oncpu) != smp_processor_id()))
 		goto out;
 
-	rb = ring_buffer_get(sampler->parent ? sampler->parent : sampler);
+	rb = ring_buffer_get(sampler);
 	if (!rb)
 		goto out;
 
@@ -6829,7 +6837,7 @@ static void perf_aux_sample_output(struct perf_event *event,
 	if (WARN_ON_ONCE(!sampler || !data->aux_size))
 		return;
 
-	rb = ring_buffer_get(sampler->parent ? sampler->parent : sampler);
+	rb = ring_buffer_get(sampler);
 	if (!rb)
 		return;