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

[Suzuki K Poulose <suzuki.poulose@arm.com>]

On 21/09/2021 16:17, Mathieu Poirier wrote:
> On Tue, Sep 21, 2021 at 02:02:31PM +0100, James Clark wrote:
>> 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(-)
> 
> 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,

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


>>
>> 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
>>