Re: [RFC] perf: need to expose sched_clock to correlate user sampleswith kernel samples

[John Stultz]

On 11/13/2012 12:58 PM, Steven Rostedt wrote:
> On Fri, 2012-11-09 at 18:04 -0800, John Stultz wrote:
> > On 10/16/2012 10:23 AM, Peter Zijlstra wrote:
> > > I've no problem with adding CLOCK_PERF (or another/better name).
> > Hrm. I'm not excited about exporting that sort of internal kernel
> > details to userland.
> >
> > The behavior and expectations from sched_clock() has changed over the
> > years, so I'm not sure its wise to export it, since we'd have to
> > preserve its behavior from then on.
> >
> > Also I worry that it will be abused in the same way that direct TSC
> > access is, where the seemingly better performance from the more
> > careful/correct CLOCK_MONOTONIC would cause developers to write fragile
> > userland code that will break when moved from one machine to the next.
> >
> > I'd probably rather perf output timestamps to userland using sane clocks
> > (CLOCK_MONOTONIC), rather then trying to introduce a new time domain to
> > userland.   But I probably could be convinced I'm wrong.
> I'm surprised that perf has its own clock anyway. But I would like to
> export the tracing clocks. We have three (well four) of them:
>
> trace_clock_local() which is defined to be a very fast clock but may not
> be synced with other cpus (basically, it just calls sched_clock).
>
> trace_clock() which is not totally serialized, but also not totally off
> (between local and global). This uses local_clock() which is the same
> thing that perf_clock() uses.
>
> trace_clock_global() which is a monotonic clock across CPUs. It's much
> slower than the above, but works well when you require synced
> timestamps.
>
> There's also trace_clock_counter() which isn't even a clock :-)  It's
> just a incremental atomic counter that goes up every time it's called.
> This is the most synced clock, but is absolutely meaningless for
> timestamps. It's just a way to show ordered events.

Oof. This is getting uglier. I'd really prefer not to expose all these 
different internal clocks out userland. Especially via clock_gettime().

thanks
-john

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