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

[Ingo Molnar <mingo@kernel.org>]

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

> On Fri, May 10, 2013 at 12:18:23PM +0200, Ingo Molnar wrote:
> > 
> > * Peter Zijlstra <peterz@infradead.org> wrote:
> > 
> > > > so you mean just detect that by opening events with increasing precise 
> > > > and see how far we could get.. could be I guess, though the 'precise' 
> > > > sysfs attribute seems more fit to me
> > > 
> > > The other way around, start at ppp end at !p, then use the one that 
> > > worked.
> > 
> > Really, instead of this silly 'probing until it works' notion, how about 
> > the radical idea that we pass to the kernel our request, and the kernel 
> > fulfills our wish to the best of its ability?
> > 
> > This could be done as a new PERF_COUNT_HW_CPU_CYCLES_PRECISE event, to 
> > which tooling could migrate, without changing existing semantics.
> > 
> > The problem with the complex probing is that it's totally unnecessary 
> > complexity that results from lack of passing the right information to the 
> > kernel. Forcing that will only hinder user-space adoption of our precise 
> > profiling facilities.
> 
> The part I have trouble with is that its a vague request and you'll get 
> a vague answer.

PERF_COUNT_HW_CPU_CYCLES_PRECISE is not a vague request at all: it means 
'get me the most precise cycles profiling available on this system'.

Thanks,

	Ingo