Re: [PATCH] coresight: Don't immediately close events that are run on invalid CPU/sink combos

On Tue, Sep 21, 2021 at 05:22:23PM +0100, Suzuki K Poulose wrote:

On 21/09/2021 16:17, Mathieu Poirier wrote:

quoted

On Tue, Sep 21, 2021 at 02:02:31PM +0100, James Clark wrote:

quoted

When a traced process runs on a CPU that can't reach the selected sink,
the event will be stopped with PERF_HES_STOPPED. This means that even if
the process migrates to a valid CPU, tracing will not resume.

This can be reproduced (on N1SDP) by using taskset to start the process
on CPU 0, and then switching it to CPU 2 (ETF 1 is only reachable from
CPU 2):

   taskset --cpu-list 0 ./perf record -e cs_etm/@tmc_etf1/ --per-thread -- taskset --cpu-list 2 ls

This produces a single 0 length AUX record, and then no more trace:

   0x3c8 [0x30]: PERF_RECORD_AUX offset: 0 size: 0 flags: 0x1 [T]

After the fix, the same command produces normal AUX records. The perf
self test "89: Check Arm CoreSight trace data recording and synthesized
samples" no longer fails intermittently. This was because the taskset in
the test is after the fork, so there is a period where the task is
scheduled on a random CPU rather than forced to a valid one.

Specifically selecting an invalid CPU will still result in a failure to
open the event because it will never produce trace:

   ./perf record -C 2 -e cs_etm/@tmc_etf0/
   failed to mmap with 12 (Cannot allocate memory)

The only scenario that has changed is if the CPU mask has a valid CPU
sink combo in it.

Testing
=======

* Coresight self test passes consistently:
   ./perf test Coresight

* CPU wide mode still produces trace:
   ./perf record -e cs_etm// -a

* Invalid -C options still fail to open:
   ./perf record -C 2,3 -e cs_etm/@tmc_etf0/
   failed to mmap with 12 (Cannot allocate memory)

* Migrating a task to a valid sink/CPU now produces trace:
   taskset --cpu-list 0 ./perf record -e cs_etm/@tmc_etf1/ --per-thread -- taskset --cpu-list 2 ls

* If the task remains on an invalid CPU, no trace is emitted:
   taskset --cpu-list 0 ./perf record -e cs_etm/@tmc_etf1/ --per-thread -- ls

Signed-off-by: James Clark <redacted>
---
  .../hwtracing/coresight/coresight-etm-perf.c  | 27 +++++++++++++++----
  1 file changed, 22 insertions(+), 5 deletions(-)

Very interesting corner case - and I like your solution.  Arnaldo, please
consider.

Reviewed-by: Mathieu Poirier <mathieu.poirier@linaro.org>

PS: This is for coresight driver, I can pick this up. Otherwise,

As yes, silly me... Sure, please add it to the tree.

Reviewed-by: Suzuki K Poulose <suzuki.poulose@arm.com>

quoted

quoted

diff --git a/drivers/hwtracing/coresight/coresight-etm-perf.c b/drivers/hwtracing/coresight/coresight-etm-perf.c
index 8ebd728d3a80..79346f0f0e0b 100644
--- a/drivers/hwtracing/coresight/coresight-etm-perf.c
+++ b/drivers/hwtracing/coresight/coresight-etm-perf.c

@@ -452,9 +452,14 @@ static void etm_event_start(struct perf_event *event, int flags)
  	 * sink from this ETM. We can't do much in this case if
  	 * the sink was specified or hinted to the driver. For
  	 * now, simply don't record anything on this ETM.
+	 *
+	 * As such we pretend that everything is fine, and let
+	 * it continue without actually tracing. The event could
+	 * continue tracing when it moves to a CPU where it is
+	 * reachable to a sink.
  	 */
  	if (!cpumask_test_cpu(cpu, &event_data->mask))
-		goto fail_end_stop;
+		goto out;
  	path = etm_event_cpu_path(event_data, cpu);
  	/* We need a sink, no need to continue without one */

@@ -466,16 +471,15 @@ static void etm_event_start(struct perf_event *event, int flags)
  	if (coresight_enable_path(path, CS_MODE_PERF, handle))
  		goto fail_end_stop;
-	/* Tell the perf core the event is alive */
-	event->hw.state = 0;
-
  	/* Finally enable the tracer */
  	if (source_ops(csdev)->enable(csdev, event, CS_MODE_PERF))
  		goto fail_disable_path;
+out:
+	/* Tell the perf core the event is alive */
+	event->hw.state = 0;
  	/* Save the event_data for this ETM */
  	ctxt->event_data = event_data;
-out:
  	return;
  fail_disable_path:

@@ -517,6 +521,19 @@ static void etm_event_stop(struct perf_event *event, int mode)
  	if (WARN_ON(!event_data))
  		return;
+	/*
+	 * Check if this ETM was allowed to trace, as decided at
+	 * etm_setup_aux(). If it wasn't allowed to trace, then
+	 * nothing needs to be torn down other than outputting a
+	 * zero sized record.
+	 */
+	if (handle->event && (mode & PERF_EF_UPDATE) &&
+	    !cpumask_test_cpu(cpu, &event_data->mask)) {
+		event->hw.state = PERF_HES_STOPPED;
+		perf_aux_output_end(handle, 0);
+		return;
+	}
+
  	if (!csdev)
  		return;

-- 
2.28.0

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