Messages in this thread Patch in this message | | | Date | Mon, 24 Jan 2022 12:49:27 +0100 | From | Peter Zijlstra <> | Subject | Re: [RFC PATCH 1/1] perf/core: Wake up parent event if inherited event has no ring buffer |
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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;
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