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

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:

quoted

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 <redacted>
Cc: Mike Leach <redacted>
Cc: Anshuman Khandual <redacted>
Cc: Leo Yan <redacted>
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.

quoted

 	}
 
 	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

quoted

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

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