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

[Ingo Molnar <mingo@kernel.org>]

* Peter Zijlstra <peterz@infradead.org> 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.

We could try and see how many people complain. It's not like it's hard to 
undo such a change of the default event?

Thanks,

	Ingo