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

[Mathieu Poirier <mathieu.poirier@linaro.org>]

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

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

> 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
> > > 
> 

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