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

[Ingo Molnar <mingo@kernel.org>]

* Peter Zijlstra <peterz@infradead.org> wrote:

> On Mon, May 13, 2013 at 09:43:13PM +0200, Ingo Molnar wrote:
> > 
> > * 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.
> 
> Ah indeed, we talked about modifying the attr structure before (error details
> or so). Did something like that ever make it in, or would this be the first
> use now?

That remained on the level of talk AFAICT.

> > 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?
> 
> I suppose so.. Alternatively we can have the PEBS event read a 'real' 
> cycles counter and weight the sample based on that. Bit cumbersome, esp 
> if you want to implement it kernel side, but it could possibly work 
> around this issue.

Looks a bit cumbersome indeed. Lets try the simpler approach and see?

Thanks,

	Ingo