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Date: Mon, 7 Jun 2021 17:27:25 +0200 From: Peter Zijlstra <peterz@...radead.org> To: Leo Yan <leo.yan@...aro.org> Cc: Arnaldo Carvalho de Melo <acme@...nel.org>, Ingo Molnar <mingo@...hat.com>, Mark Rutland <mark.rutland@....com>, Alexander Shishkin <alexander.shishkin@...ux.intel.com>, Jiri Olsa <jolsa@...hat.com>, Namhyung Kim <namhyung@...nel.org>, Thomas Gleixner <tglx@...utronix.de>, x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>, Mathieu Poirier <mathieu.poirier@...aro.org>, Suzuki K Poulose <suzuki.poulose@....com>, Mike Leach <mike.leach@...aro.org>, Adrian Hunter <adrian.hunter@...el.com>, Andi Kleen <ak@...ux.intel.com>, linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org, coresight@...ts.linaro.org, linux-arm-kernel@...ts.infradead.org Subject: Re: [PATCH v2 1/8] perf/ring_buffer: Add comment for barriers on AUX ring buffer On Wed, Jun 02, 2021 at 06:30:00PM +0800, Leo Yan wrote: > AUX ring buffer applies almost the same barriers as perf ring buffer, > but there has an exception for ordering between writing the AUX trace > data and updating user_page::aux_head. > > This patch adds comment for how to use the barriers on AUX ring buffer, > and gives comment to ask the drivers to flush the trace data into AUX > ring buffer prior to updating user_page::aux_head. > > Signed-off-by: Leo Yan <leo.yan@...aro.org> Acked-by: Peter Zijlstra (Intel) <peterz@...radead.org> > --- > kernel/events/ring_buffer.c | 9 +++++++++ > 1 file changed, 9 insertions(+) > > diff --git a/kernel/events/ring_buffer.c b/kernel/events/ring_buffer.c > index 52868716ec35..5cf6579be05e 100644 > --- a/kernel/events/ring_buffer.c > +++ b/kernel/events/ring_buffer.c > @@ -509,6 +509,15 @@ void perf_aux_output_end(struct perf_output_handle *handle, unsigned long size) > perf_event_aux_event(handle->event, aux_head, size, > handle->aux_flags); > > + /* > + * See perf_output_put_handle(), AUX ring buffer applies the same > + * barrier pairing as the perf ring buffer; except for B, since > + * AUX ring buffer is written by hardware trace, we cannot simply > + * use the generic memory barrier (like smp_wmb()) prior to update > + * user_page::aux_head, the hardware trace driver takes the > + * responsibility to ensure the trace data has been flushed into > + * the AUX buffer before calling perf_aux_output_end(). > + */ > WRITE_ONCE(rb->user_page->aux_head, rb->aux_head); > if (rb_need_aux_wakeup(rb)) > wakeup = true; > -- > 2.25.1 >
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