Re: [PATCH v2 07/10] coresight: trbe: Do not truncate buffer on IRQ

Hi Suzuki,

On Fri, 23 Jul 2021 at 13:46, Suzuki K Poulose suzuki.poulose@arm.com 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.

What is the user visible behaviour when using COLLISION? The TRUNCATE warning is at least accurate - even if the KVM thing is something of a red herring. It is easier to explain a "scary" warning, than try to debug someones problems if perf is silent or misleading when using the COLLISION flag.

Regards

Mike

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

---

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 503bea0137ae..d50f142e86d1 100644 --- a/drivers/hwtracing/coresight/coresight-trbe.c +++ b/drivers/hwtracing/coresight/coresight-trbe.c @@ -615,7 +615,7 @@ 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);
    }
  
    offset = write - base;
  

@@ -708,7 +708,7 @@ static void trbe_handle_overflow(struct perf_output_handle *handle) * collection upon the WRAP event, without stopping the source. */ perf_aux_output_flag(handle, PERF_AUX_FLAG_CORESIGHT_FORMAT_RAW |

•                                PERF_AUX_FLAG_TRUNCATED);
  

•                                PERF_AUX_FLAG_COLLISION);
    perf_aux_output_end(handle, size);
    event_data = perf_aux_output_begin(handle, event);
    if (!event_data) {
  

-- 2.24.1