Re: [PATCH v4 11/22] perf: add ITRACE_START record to indicate that tracing has started

[Alexander Shishkin]

Peter Zijlstra <peterz@xxxxxxxxxxxxx> writes:

> On Wed, Aug 20, 2014 at 03:36:08PM +0300, Alexander Shishkin wrote:
>> For counters such as instruction tracing, it is useful for the decoder
>> to know which tasks are running when the event is first scheduled in,
>> before the first sched_switch.
>> 
>> To single out such instruction tracing pmus, this patch alse introduces
>> ITRACE PMU capability.
>
> You forgot to tell why. Also explain why.
>
> The next context switch event will also tell you the previous task. So
> this seems like superfluous information. Also if you really want this
> you could simply emit an empty AUX record I suppose, no need to create
> yet another record type.

Before the first sched_switch we don't actually know what pid/tid is
running, so we can't make sense of the beginning of the trace without
some event reordering trickery. We can use AUX for that as well, I guess
this might be another use case for those flags in the AUX
record. Changelog, got it.

Adrian and Arnaldo cc'ed.

Cheers,
--
Alex
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