linux-kernel - Re: [PATCH v1 4/6] perf: Allow using AUX data in perf samples

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Date:   Thu, 21 Jun 2018 22:16:32 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Alexander Shishkin <alexander.shishkin@...ux.intel.com>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...hat.com>,
        linux-kernel@...r.kernel.org, jolsa@...hat.com
Subject: Re: [PATCH v1 4/6] perf: Allow using AUX data in perf samples

On Tue, Jun 19, 2018 at 01:47:25PM +0300, Alexander Shishkin wrote:
> On Thu, Jun 14, 2018 at 10:20:22PM +0200, Peter Zijlstra wrote:

> > More yuck...
> > 
> > You rreally should not be calling these pmu::methods, they're meant to
> > be used from _interrupt_ not NMI context. Using them like this is asking
> > for tons of trouble.
> 
> Right, the SW stuff may then race with event_function_call() stuff. Hmm.
> For the HW stuff, I'm hoping that some kind of a sleight of hand may
> suffice. Let me think some more.

I currently don't see how the SW driven snapshot can ever work, see my
comment on the last patch.

> > Why can't you just snapshot the current location and let the thing
> > 'run' ?
> 
> Because the buffer will overwrite itself and the location will be useless.

Not if it's large enough ;-)

> We don't write the AUX data out in this 'mode' at all, only the samples,
> which allows for much less data in the resulting perf.data, less work for
> the consumer, less IO bandwidth etc, and as a bonus, no AUX-related
> interrupts.
> 
> But actually, even to snapshot the location we need to stop the event.

Maybe new methods that should only be called from NMI context?