Re: [PATCH v3 07/10] coresight: trbe: Do not truncate buffer on IRQ

From: Mathieu Poirier <mathieu.poirier@linaro.org>
To: Suzuki K Poulose <suzuki.poulose@arm.com>
Cc: coresight@lists.linaro.org, linux-arm-kernel@lists.infradead.org,
	anshuman.khandual@arm.com, mike.leach@linaro.org,
	leo.yan@linaro.org, tamas.zsoldos@arm.com,
	jinlmao@qti.qualcomm.com, al.grant@arm.com, denik@google.com,
	James Clark <james.clark@arm.com>
Subject: Re: [PATCH v3 07/10] coresight: trbe: Do not truncate buffer on IRQ
Date: Tue, 21 Sep 2021 11:50:15 -0600	[thread overview]
Message-ID: <20210921175015.GE2059841@p14s> (raw)
In-Reply-To: <20210921174110.GD2059841@p14s>

On Tue, Sep 21, 2021 at 11:41:10AM -0600, Mathieu Poirier wrote:
> On Tue, Sep 14, 2021 at 11:26:38AM +0100, Suzuki K Poulose wrote:
> > The TRBE driver marks the AUX buffer as TRUNCATED when we get an IRQ
> > on FILL event. This has rather unwanted side-effect of the event
> > being disabled when there may be more space in the ring buffer.
> > 
> > So, instead of TRUNCATE we need a different flag to indicate
> > that the trace may have lost a few bytes (i.e from the point of
> > generating the FILL event until the IRQ is consumed). Anyways, the
> > userspace must use the size from RECORD_AUX headers to restrict
> > the "trace" decoding.
> > 
> > Using PARTIAL flag causes the perf tool to generate the
> > following warning:
> > 
> >   Warning:
> >   AUX data had gaps in it XX times out of YY!
> > 
> >   Are you running a KVM guest in the background?
> > 
> > which is pointlessly scary for a user. The other remaining options
> > are :
> >   - COLLISION - Use by SPE to indicate samples collided
> >   - Add a new flag - Specifically for CoreSight, doesn't sound
> >     so good, if we can re-use something.
> > 
> > Given that we don't already use the "COLLISION" flag, the above
> > behavior can be notified using this flag for CoreSight.
> > 
> > Cc: Mathieu Poirier <mathieu.poirier@linaro.org>
> > Cc: James Clark <james.clark@arm.com>
> > Cc: Mike Leach <mike.leach@linaro.org>
> > Cc: Anshuman Khandual <anshuman.khandual@arm.com>
> > Cc: Leo Yan <leo.yan@linaro.org>
> > Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
> > ---
> > Changes since v2:
> >   - The perf tool patch for reporting collisions is queued.
> > ---
> >  drivers/hwtracing/coresight/coresight-trbe.c | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> > 
> > diff --git a/drivers/hwtracing/coresight/coresight-trbe.c b/drivers/hwtracing/coresight/coresight-trbe.c
> > index de99dd0aecd3..a1a15fa6c4ae 100644
> > --- a/drivers/hwtracing/coresight/coresight-trbe.c
> > +++ b/drivers/hwtracing/coresight/coresight-trbe.c
> > @@ -612,7 +612,7 @@ static unsigned long arm_trbe_update_buffer(struct coresight_device *csdev,
> >  		 * for correct size. Also, mark the buffer truncated.
> >  		 */
> >  		write = get_trbe_limit_pointer();
> > -		perf_aux_output_flag(handle, PERF_AUX_FLAG_TRUNCATED);
> > +		perf_aux_output_flag(handle, PERF_AUX_FLAG_COLLISION);
> 
> The rational in the changelog should be added here as a comment.
> 
> >  	}
> >  
> >  	offset = write - base;
> > @@ -705,7 +705,7 @@ static void trbe_handle_overflow(struct perf_output_handle *handle)
> >  	 * Mark the buffer as truncated, as we have stopped the trace
> >  	 * collection upon the WRAP event, without stopping the source.
> >  	 */
> > -	perf_aux_output_flag(handle, PERF_AUX_FLAG_TRUNCATED);
> > +	perf_aux_output_flag(handle, PERF_AUX_FLAG_COLLISION);
> 
> Same.

... and I'm out of time for today.  I will continue tomorrow.

Thanks,
Mathieu

> 
> >  	perf_aux_output_end(handle, size);
> >  	event_data = perf_aux_output_begin(handle, event);
> >  	if (!event_data) {
> > -- 
> > 2.24.1
> > 

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