Messages in this thread Patch in this message | | | Subject | Re: [RFC PATCH 1/1] perf/core: Wake up parent event if inherited event has no ring buffer | From | James Clark <> | Date | Mon, 24 Jan 2022 14:58:18 +0000 |
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On 24/01/2022 11:49, Peter Zijlstra wrote: > 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?
Yes I tested this and it also works. There is one other suspicious access of ->rb followed by if(rb) here in perf_poll(), but maybe it works out ok?
mutex_lock(&event->mmap_mutex); rb = event->rb; if (rb) events = atomic_xchg(&rb->poll, 0);
We also have a Perf self test that covers this failure for Arm SPE now, I'm not sure if I should post that separately or with your new version of this fix?
Thanks James > > > --- > 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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