Re: [PATCH v3 4/6] coresight: perf: Support AUX trace pause and resume

Hi Leo,

On Tue, 11 Mar 2025 at 17:05, Leo Yan leo.yan@arm.com wrote:

This commit supports AUX trace pause and resume in a perf session for Arm CoreSight.

First, we need to decide which flag can indicate the CoreSight PMU event has started. The 'event->hw.state' cannot be used for this purpose because its initial value and the value after hardware trace enabling are both 0.

On the other hand, the context value 'ctxt->event_data' stores the ETM private info. This pointer is valid only when the PMU event has been enabled. It is safe to permit AUX trace pause and resume operations only when it is not a NULL pointer.

To achieve fine-grained control of the pause and resume, only the tracer is disabled and enabled. This avoids the unnecessary complexity and latency caused by manipulating the entire link path.

Signed-off-by: Leo Yan leo.yan@arm.com

.../hwtracing/coresight/coresight-etm-perf.c | 45 ++++++++++++++++++- 1 file changed, 44 insertions(+), 1 deletion(-)

diff --git a/drivers/hwtracing/coresight/coresight-etm-perf.c b/drivers/hwtracing/coresight/coresight-etm-perf.c index f4cccd68e625..2dcf1809cb7f 100644 --- a/drivers/hwtracing/coresight/coresight-etm-perf.c +++ b/drivers/hwtracing/coresight/coresight-etm-perf.c @@ -365,6 +365,18 @@ static void *etm_setup_aux(struct perf_event *event, void **pages, continue; }

•           /*
  

•            * If AUX pause feature is enabled but the ETM driver does not
  

•            * support the operations, clear this CPU from the mask and
  

•            * continue to next one.
  

•            */
  

•           if (event->attr.aux_start_paused &&
  

•               (!source_ops(csdev)->pause_perf || !source_ops(csdev)->resume_perf)) {
  

•                   dev_err_once(&csdev->dev, "AUX pause is not supported.\n");
  

•                   cpumask_clear_cpu(cpu, mask);
  

•                   continue;
  

•           }
  

•           /*
             * No sink provided - look for a default sink for all the ETMs,
             * where this event can be scheduled.
  

@@ -450,6 +462,15 @@ static void *etm_setup_aux(struct perf_event *event, void **pages, goto out; }

+static int etm_event_resume(struct coresight_device *csdev,

•                        struct etm_ctxt *ctxt)
  

+{

•   if (!ctxt->event_data)
  

•           return 0;
  

•   return coresight_resume_source(csdev);
  

+}

static void etm_event_start(struct perf_event *event, int flags) { int cpu = smp_processor_id(); @@ -463,6 +484,14 @@ static void etm_event_start(struct perf_event *event, int flags) if (!csdev) goto fail;

Is it possible here that the first call to etm_event_start() also has the PERF_EF_RESUME flag set? If so it looks like we need to fall through and do a "normal" start to get all the ctxt->event_data set up.

•   if (flags & PERF_EF_RESUME) {
  

•           if (etm_event_resume(csdev, ctxt) < 0) {
  

•                   dev_err(&csdev->dev, "Failed to resume ETM event.\n");
  

•                   goto fail;
  

•           }
  

•           return;
  

•   }
  

•   /* Have we messed up our tracking ? */
    if (WARN_ON(ctxt->event_data))
            goto fail;
  

@@ -545,6 +574,16 @@ static void etm_event_start(struct perf_event *event, int flags) return; }

+static void etm_event_pause(struct coresight_device *csdev,

•                       struct etm_ctxt *ctxt)
  

+{

•   if (!ctxt->event_data)
  

•           return;
  

•   /* Stop tracer */
  

•   coresight_pause_source(csdev);
  

+}

static void etm_event_stop(struct perf_event *event, int mode) { int cpu = smp_processor_id(); @@ -555,6 +594,9 @@ static void etm_event_stop(struct perf_event *event, int mode) struct etm_event_data *event_data; struct coresight_path *path;

•   if (mode & PERF_EF_PAUSE)
  

•           return etm_event_pause(csdev, ctxt);
  

•   /*
     * If we still have access to the event_data via handle,
     * confirm that we haven't messed up the tracking.
  

@@ -899,7 +941,8 @@ int __init etm_perf_init(void) int ret;

    etm_pmu.capabilities            = (PERF_PMU_CAP_EXCLUSIVE |

•                                      PERF_PMU_CAP_ITRACE);
  

•                                      PERF_PMU_CAP_ITRACE |
  

•                                      PERF_PMU_CAP_AUX_PAUSE);
  
    etm_pmu.attr_groups             = etm_pmu_attr_groups;
    etm_pmu.task_ctx_nr             = perf_sw_context;
  

-- 2.34.1

If the possible issue above is prevented by perf internals

Reviewed-by: Mike Leach mike.leach@linaro.org

-- Mike Leach Principal Engineer, ARM Ltd. Manchester Design Centre. UK