Messages in this thread | | | Date | Tue, 14 May 2013 10:37:53 +0200 | From | Ingo Molnar <> | Subject | Re: [PATCH 0/9] perf: Adding better precise_ip field handling |
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* 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
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