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

James Clark
2021-09-21T13:02:47Z

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 <james.clark@arm.com>
---
.../hwtracing/coresight/coresight-etm-perf.c | 27 +++++++++++++++----
1 file changed, 22 insertions(+), 5 deletions(-)

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