On 20/12/2023 16:16, Adrian Hunter wrote:

On 20/12/23 17:54, James Clark wrote:

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On 08/12/2023 17:24, Adrian Hunter wrote:

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Hardware traces, such as instruction traces, can produce a vast amount of trace data, so being able to reduce tracing to more specific circumstances can be useful.

The ability to pause or resume tracing when another event happens, can do that.

Add ability for an event to "pause" or "resume" AUX area tracing.

Add aux_pause bit to perf_event_attr to indicate that, if the event happens, the associated AUX area tracing should be paused. Ditto aux_resume. Do not allow aux_pause and aux_resume to be set together.

Add aux_start_paused bit to perf_event_attr to indicate to an AUX area event that it should start in a "paused" state.

Add aux_paused to struct perf_event for AUX area events to keep track of the "paused" state. aux_paused is initialized to aux_start_paused.

Add PERF_EF_PAUSE and PERF_EF_RESUME modes for ->stop() and ->start() callbacks. Call as needed, during __perf_event_output(). Add aux_in_pause_resume to struct perf_buffer to prevent races with the NMI handler. Pause/resume in NMI context will miss out if it coincides with another pause/resume.

To use aux_pause or aux_resume, an event must be in a group with the AUX area event as the group leader.

Example (requires Intel PT and tools patches also):

$ perf record --kcore -e '{intel_pt/aux-start-paused/k,syscalls:sys_enter_newuname/aux-resume/,syscalls:sys_exit_newuname/aux-pause/}' uname

I think it might be useful to have an aux-toggle option as well, and then you could do sampling if you put it on a PMU counter with an interval. Unless you can make two events for the same counter with different intervals, and one does resume and the other does pause? I'm not sure if that would work?

There is already ->snapshot_aux() for sampling. Is that what you mean?

I suppose that mostly handles that use case yes. Although there are some slight differences. It looks like for SAMPLE_AUX, the buffer size for each sample is fixed and limited to 16 bits in size, whereas between pause and resume you could potentially have multiple buffers delivered to userspace of any size.

And it looks like SAMPLE_AUX would leave the trace running even when no samples were being collected. I suppose you might not want to consume the memory bandwidth and turn the trace off between samples, which pause/resume does. Especially if you intend to have long periods between the samples.

I think if it did turn out to be useful the toggle function can easily be added later, so I don't intend this comment to be a blocking one.

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Other than that it looks ok. I got Coresight working with a couple of changes to what you posted on here, but that can always be done more thoroughly later if we leave PERF_PMU_CAP_AUX_PAUSE off Coresight for now.

Thanks a lot for looking at this!