Re: [PATCH RFC V2 1/4] perf/core: Add aux_pause, aux_resume, aux_start_paused

[Adrian Hunter <adrian.hunter@intel.com>]

On 20/12/23 17:54, James Clark wrote:
> 
> 
> On 08/12/2023 17:24, Adrian Hunter wrote:
>> 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?

> 
> 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!