Re: [PATCH 0/9] perf: Adding better precise_ip field handling

[Ingo Molnar]

* Stephane Eranian <eranian@xxxxxxxxxx> wrote:

> On Mon, May 13, 2013 at 9:43 PM, Ingo Molnar <mingo@xxxxxxxxxx> wrote:
> >
> > * Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> >
> >> On Sat, May 11, 2013 at 09:50:08AM +0200, Ingo Molnar wrote:
> >> > That's really a red herring: there's absolutely no reason why the
> >> > kernel could not pass back the level of precision it provided.
> >>
> >> All I've been saying is that doing random precision without feedback is
> >> confusing.
> >
> > I agree with that.
> >
> >> We also don't really have a good feedback channel for this kind of
> >> thing. The best I can come up with is tagging each and every sample with
> >> the quality it represents. I think we can do with only one extra
> >> PERF_RECORD_MISC bit, but it looks like we're quickly running out of
> >> those things.
> >
> > Hm, how about passing precision back to user-space at creation time, in
> > the perf_attr data structure? There's no need to pass it back in every
> > sample, precision will not really change during the life-time of an event.
> >
> >> But I think the biggest problem is PEBS's inability do deal with REP
> >> prefixes; see this email from Stephane:
> >> https://lkml.org/lkml/2011/2/1/177
> >>
> >> It is really unfortunate for PEBS to have such a side-effect; but it
> >> makes all memset/memcpy/memmove things appear like they have no cost.
> >> I'm very sure that will surprise a number of people.
> >
> > I'd expect PEBS to get gradually better.
> >
> > Note that at least for user-space, REP MOVS is getting rarer. libc uses
> > SSE based memcpy/memset variants - which is not miscounted by PEBS. The
> > kernel still uses REP MOVS - but it's a special case because it cannot
> > cheaply use vector registers.
> >
> > The vast majority of code gets measured by cycles:pp more accurately than
> > cycles.
> >
> I don't understand how you come to that conclusion. [...]

By frequently looking at cycles:pp output.

> [...] I can show you simple examples where this is not true at all (even 
> without rep mov).

That would be useful if there's any practical problem with cycles:pp. In 
terms of profiling typical kernel and user space functions it does appear 
to work very well.

Thanks,

	Ingo
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