linux-kernel - Re: [PATCH v3 4/6] coresight: perf: Support AUX trace pause and resume

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Message-ID: <CAJ9a7VhOg6emwDMa+rZM4TqZcMPY=fYSxzb4EQVxOhY0T-tg_Q@mail.gmail.com>
Date: Wed, 2 Apr 2025 13:56:10 +0100
From: Mike Leach <mike.leach@...aro.org>
To: Leo Yan <leo.yan@....com>
Cc: Suzuki K Poulose <suzuki.poulose@....com>, James Clark <james.clark@...aro.org>, 
	Jonathan Corbet <corbet@....net>, Alexander Shishkin <alexander.shishkin@...ux.intel.com>, 
	coresight@...ts.linaro.org, linux-arm-kernel@...ts.infradead.org, 
	linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 4/6] coresight: perf: Support AUX trace pause and resume

Hi Leo

On Wed, 2 Apr 2025 at 13:31, Leo Yan <leo.yan@....com> wrote:
>
> Hi Mike,
>
> On Wed, Apr 02, 2025 at 09:45:20AM +0100, Mike Leach wrote:
>
> [...]
>
> > > > >  static void etm_event_start(struct perf_event *event, int flags)
> > > > >  {
> > > > >         int cpu = smp_processor_id();
> > > > > @@ -463,6 +484,14 @@ static void etm_event_start(struct perf_event *event, int flags)
> > > > >         if (!csdev)
> > > > >                 goto fail;
> > > > >
> > > >
> > > > Is it possible here that the first call to etm_event_start() also has
> > > > the PERF_EF_RESUME flag set?
> > >
> > > The first call has a flow below, using flag 0 but not PERF_EF_RESUME.
> > >
> > >   etm_event_add()
> > >     `>  etm_event_start(event, 0);
> > >
> >
> > When I looked at the vague comments in the perf source - it seemed to
> > imply that ->start() calls could overlap - so the associated event
> > that resumes trace could occur at the same time as the initialising
> > start from paused for the trace operations.
>
> Good point.  A subtle but important thing is the 'cs_etm' event must be
> an event group leader, otherwise, the tool reports error:
>
>   # perf record -m,64M -e cycles/aux-action=pause,period=10000000/ \
>       -e cycles/aux-action=resume,period=10/ \
>       -e cs_etm/aux-action=start-paused/u -- /mnt/sort
>   Events with aux-action must have AUX area event group leader
>
> If the 'cs_etm' event is the event group leader, it will be always
> enabled ahead other PMU events.  So etm_event_start(event, 0) is
> invoked prior to PMU events enabling.  As a result, this can avoid the
> race condition you mentioned.
>
> I confirmed with ftrace log:
>
>   sort-901     [005] d..3.  1033.827186: etm_event_add <-event_sched_in
>   sort-901     [005] d..3.  1033.827187: etm_event_start <-etm_event_add
>   sort-901     [005] d..3.  1033.827283: armpmu_add <-event_sched_in
>   sort-901     [005] d..3.  1033.827287: armpmu_start <-armpmu_add
>   sort-901     [005] d..3.  1033.827288: armpmu_event_set_period <-armpmu_start
>   sort-901     [005] d..3.  1033.827292: armpmu_add <-event_sched_in
>   sort-901     [005] d..3.  1033.827293: armpmu_start <-armpmu_add
>   sort-901     [005] d..3.  1033.827294: armpmu_event_set_period <-armpmu_start
>   sort-901     [005] d..3.  1033.827298: armpmu_filter <-visit_groups_merge.constprop.0.isra.0
>   sort-901     [005] d..3.  1033.827298: armpmu_enable <-perf_pmu_enable
>   sort-901     [005] d..3.  1033.827301: armpmu_enable <-perf_pmu_enable
>   sort-901     [005] d.h1.  1033.827304: armpmu_dispatch_irq <-__handle_irq_event_percpu
>   sort-901     [005] d.h1.  1033.827306: armpmu_event_update <-armv8pmu_handle_irq
>   sort-901     [005] d.h1.  1033.827308: armpmu_event_set_period <-armv8pmu_handle_irq
>   sort-901     [005] d.h..  1033.827322: perf_event_aux_pause: event=ffff000207503e40 pause=0
>
> > If we are guaranteed this cannot happen then we are good to go!
>
> Now I think we are safe, right?  ;)
>
> Thanks,
> Leo

I'd agree with that!

Mike

-- 
Mike Leach
Principal Engineer, ARM Ltd.
Manchester Design Centre. UK