Re: [PATCH] perf: POSIX CLOCK_PERF to report current time value

[Ingo Molnar]

* David Ahern <dsahern@xxxxxxxxx> wrote:

> From: Pawel Moll <pawel.moll@xxxxxxx>
> 
> To co-relate user space events with the perf events stream
> a current (as in: "what time(stamp) is it now?") time value
> must be made available.

So I'm wondering about your earlier approach posted here:

  https://lkml.org/lkml/2011/6/7/636

I'd modify that patch the following way: instead of tracking each 
separate reason, perhaps only track timekeeping_update().

Such a tracepoint, combined with PERF_SAMPLE_TIME, would very 
accurately track external changes to GTOD (xtime).

That leaves us with tracking/correlating the regular flow of time, 
which could be achieved by another tracepoint in:

   kernel/time/timekeeping.c::do_timer()

So only two new tracepoints are needed AFAICS - and the tracepoints 
would obviously be useful for other (debugging) purposes as well. 
Would that solve the wall-clock correlation problem adequately?

Thanks,

	Ingo
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