Thread (55 messages) 55 messages, 4 authors, 2025-12-16

Re: [PATCH 08/19] coresight: trbe: Use PERF_AUX_FLAG_PARTIAL instead of PERF_AUX_FLAG_COLLISION

From: Anshuman Khandual <hidden>
Date: 2025-12-05 04:28:12
Also in: linux-perf-users, lkml

On 01/12/25 4:51 PM, Leo Yan wrote:
quoted hunk ↗ jump to hunk
When tracing stops, there is no collision with other samples, so using
PERF_AUX_FLAG_COLLISION does not accurately reflect the trace state and
may mislead userspace.

Use PERF_AUX_FLAG_PARTIAL instead to indicate that tracing stopped and
the record may contain gaps.

Signed-off-by: Leo Yan <leo.yan@arm.com>
---
 drivers/hwtracing/coresight/coresight-trbe.c | 19 ++++++-------------
 1 file changed, 6 insertions(+), 13 deletions(-)
diff --git a/drivers/hwtracing/coresight/coresight-trbe.c b/drivers/hwtracing/coresight/coresight-trbe.c
index b06885a08e082fd34f68d9588518807b5c47c86e..0caa4a6b437a3aa39fc6bcc72a23711b54f7c598 100644
--- a/drivers/hwtracing/coresight/coresight-trbe.c
+++ b/drivers/hwtracing/coresight/coresight-trbe.c
@@ -669,21 +669,14 @@ static enum trbe_fault_action trbe_get_fault_act(struct perf_output_handle *hand
 	}
 
 	/*
-	 * Mark the buffer to indicate that there was a WRAP event by
-	 * setting the COLLISION flag. This indicates to the user that
-	 * the TRBE trace collection was stopped without stopping the
-	 * ETE and thus there might be some amount of trace that was
-	 * lost between the time the WRAP was detected and the IRQ
-	 * was consumed by the CPU.
-	 *
-	 * Setting the TRUNCATED flag would move the event to STOPPED
-	 * state unnecessarily, even when there is space left in the
-	 * ring buffer. Using the COLLISION flag doesn't have this side
-	 * effect. We only set TRUNCATED flag when there is no space
-	 * left in the ring buffer.
+	 * Mark the buffer to indicate that the trace is stopped by setting
+	 * the PARTIAL flag. This indicates to the user that the TRBE trace
+	 * collection was stopped without stopping the ETE and thus there
+	 * might be some amount of trace that was lost between the time the
+	 * TRBE event was detected and the IRQ was consumed by the CPU.
 	 */
 	if (!is_trbe_running(trbsr))
-		perf_aux_output_flag(handle, PERF_AUX_FLAG_COLLISION);
+		perf_aux_output_flag(handle, PERF_AUX_FLAG_PARTIAL);
This is interesting. So there is a no possibility for the records to be
overridden in the perf ring buffer and incomplete record is all that can
happen when TRBE is stopped, without stopping the ETE first ? Could both
of these scenarios might be possible as well ?
 
 	if (is_trbe_wrap(trbsr))
 		return TRBE_FAULT_ACT_WRAP;
  
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