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    SubjectRe: [PATCH 3/6] perf record: add time-of-day option
    Em Tue, Mar 01, 2011 at 02:07:50PM -0300, Arnaldo Carvalho de Melo escreveu:
    > Em Tue, Mar 01, 2011 at 09:45:27AM -0700, David Ahern escreveu:
    > > On 03/01/2011 09:37 AM, Peter Zijlstra wrote:
    > > > On Tue, 2011-03-01 at 09:09 -0700, David Ahern wrote:
    > > >>
    > > >> I did not code it create samples of a realtime-clock event; I coded it
    > > >> to add an event to the stream on demand -- whatever event the fd is
    > > >> tied to.
    > > >>
    > > >>
    > > > Well yes, but aside from you and the realtime clock stuff, who'd ever
    > > > want to do this particular thing?
    > >
    > > One example would be a system watchdog that decided for some reason or
    > > another to force an event sample because of some event it detected.
    > >
    > > >
    > > > I'd much rather expose the whole
    > > > local_clock()/perf_clock()/trace_clock() (should all be the same anyway)
    > > > as a posix clock using CLOCK_TRACING or whatever would be an appropriate
    > > > name.
    > > >
    > > > [ Since the whole thing is NMI safe it should be well possible to make a
    > > > VDSO version as well. ]
    > > >
    > > > Anyway, once its visible as a posix clock you can sync up from
    > > > userspace. And this clock is indeed wanted for other things too, like
    > > > user-space tracing etc.
    > > >
    > >
    > > And for some silly reason I sense a time trap here (pun intended) .....
    > >
    > > So your pushback is:
    > > 1. throw out the realtime-clock event patch,
    >
    > Nope
    >
    > > 2. add a new CLOCK_TRACING type to clock_gettime with VDSO hook,
    >
    > Yes
    >
    > > 3. and use a synthesized event from userspace for snapshotting
    > > perf_clock to time-of-day -- similar to what I have now, but one that
    > > will guarantee a time-of-day to perf_clock correlation (versus the
    > > current one which hopes that perf_clock is the monotonic clock).
    >
    > Nope.

    I see, yeah, the very first CLOCK_REALTIME sample would be synthesized
    from the return of clock_gettime(CLOCK_TRACING), all the others would
    come in the event stream.

    > My understanding was that for the initial gettime you use
    > clock_gettime(), and then you get CLOCK_REALTIME events in the stream
    > for resynch, etc.
    >
    > Peter?
    >
    > - Arnaldo


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