* Ingo Molnar <[email protected]> wrote:
>
> * David Ahern <[email protected]> wrote:
>
> > From: Pawel Moll <[email protected]>
> >
> > 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?
I forgot to mention what data the new tracepoints would trace:
- timekeeping_update() would trace the 'before' and 'after'
(==current) GTOD values
- do_timer() would trace 'ticks' and the current GTOD value.
'GTOD value' is what gettimeofday() would return, which in the
do_timer() case ought to be something like this:
tk->xtime_sec
(tk->xtime_nsec >> tk->shift) + get_arch_timeoffset()
(Note that this is simple and fast as there's no need to read the
clock once again, it was just read.)
Thanks,
Ingo
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