Will Deacon <will.dea...@arm.com> writes: > The aux_watermark member of struct ring_buffer represents the period (in > terms of bytes) at which wakeup events should be generated when data is > written to the aux buffer in non-snapshot mode. On hardware that cannot > generate an interrupt when the aux_head reaches an arbitrary wakeup index
Curious: how do you support non-snapshot trace collection on such hardware? > (such as ARM SPE), the aux_head sampled from handle->head in > perf_aux_output_{skip,end} may in fact be past the wakeup index. This I think this is also true of hw where the interrupt is not precise. Thanks for looking at this. > can lead to wakeup slowly falling behind the head. For example, consider > the case where hardware can only generate an interrupt on a page-boundary > and the aux buffer is initialised as follows: > > // Buffer size is 2 * PAGE_SIZE > rb->aux_head = rb->aux_wakeup = 0 > rb->aux_watermark = PAGE_SIZE / 2 > > following the first perf_aux_output_begin call, the handle is > initialised with: > > handle->head = 0 > handle->size = 2 * PAGE_SIZE > handle->wakeup = PAGE_SIZE / 2 > > and the hardware will be programmed to generate an interrupt at > PAGE_SIZE. > > When the interrupt is raised, the hardware head will be at PAGE_SIZE, > so calling perf_aux_output_end(handle, PAGE_SIZE) puts the ring buffer > into the following state: > > rb->aux_head = PAGE_SIZE > rb->aux_wakeup = PAGE_SIZE / 2 > rb->aux_watermark = PAGE_SIZE / 2 > > and then the next call to perf_aux_output_begin will result in: > > handle->head = handle->wakeup = PAGE_SIZE > > for which the semantics are unclear and, for a smaller aux_watermark > (e.g. PAGE_SIZE / 4), then the wakeup would in fact be behind head at > this point. > > This patch fixes the problem by rounding down the aux_head (as sampled > from the handle) to the nearest aux_watermark boundary when updating > rb->aux_wakeup, therefore taking into account any overruns by the > hardware. Let's add a small comment to the @aux_wakeup field definition? Other than that, Acked-by: Alexander Shishkin <alexander.shish...@linux.intel.com> > > Cc: Alexander Shishkin <alexander.shish...@linux.intel.com> > Cc: Peter Zijlstra <pet...@infradead.org> > Reported-by: Mark Rutland <mark.rutl...@arm.com> > Signed-off-by: Will Deacon <will.dea...@arm.com> > --- > kernel/events/ring_buffer.c | 4 ++-- > 1 file changed, 2 insertions(+), 2 deletions(-) > > diff --git a/kernel/events/ring_buffer.c b/kernel/events/ring_buffer.c > index 330df5a7f762..8e511e52fc1b 100644 > --- a/kernel/events/ring_buffer.c > +++ b/kernel/events/ring_buffer.c > @@ -455,7 +455,7 @@ void perf_aux_output_end(struct perf_output_handle > *handle, unsigned long size) > > if (aux_head - rb->aux_wakeup >= rb->aux_watermark) { > wakeup = true; > - rb->aux_wakeup += rb->aux_watermark; > + rb->aux_wakeup = rounddown(aux_head, rb->aux_watermark); > } > > if (wakeup) { > @@ -490,7 +490,7 @@ int perf_aux_output_skip(struct perf_output_handle > *handle, unsigned long size) > aux_head = rb->user_page->aux_head = rb->aux_head; > if (aux_head - rb->aux_wakeup >= rb->aux_watermark) { > perf_output_wakeup(handle); > - rb->aux_wakeup += rb->aux_watermark; > + rb->aux_wakeup = rounddown(aux_head, rb->aux_watermark); > handle->wakeup = rb->aux_wakeup + rb->aux_watermark; > } > > -- > 2.1.4