Commit 8c60acbc authored Sep 22, 2021 by James Clark Committed by Mathieu Poirier Oct 27, 2021

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



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

Reviewed-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Signed-off-by: James Clark <james.clark@arm.com>
Link: https://lore.kernel.org/r/20210922125144.133872-2-james.clark@arm.com


Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>

parent 0abd0762

drivers/hwtracing/coresight/coresight-etm-perf.c

+23 −6

Original line number	Diff line number	Diff line
		@@ -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:
		@@ -485,7 +489,7 @@ static void etm_event_start(struct perf_event *event, int flags)
		perf_aux_output_end(handle, 0);
		fail:
		event->hw.state = PERF_HES_STOPPED;
		goto out;
		return;
		}

		static void etm_event_stop(struct perf_event *event, int mode)
		@@ -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;