Re: [patch 0/2] NOHZ vs. profile/oprofile v2

Ingo Molnar
2009-06-22T15:29:56Z

* Martin Schwidefsky <schwidefsky@de.ibm.com> wrote:

> On Mon, 22 Jun 2009 17:05:53 +0200
> Ingo Molnar <mingo@elte.hu> wrote:
>
> >
> > * Martin Schwidefsky <schwidefsky@de.ibm.com> wrote:
> >
> > > On Mon, 22 Jun 2009 16:41:10 +0200
> > > Ingo Molnar <mingo@elte.hu> wrote:
> > >
> > > > Hm, this is rather ugly. Why not use hrtimers like 'perf' does when
> > > > it fallback-samples based on the timer tick?
> > > >
> > > > That method has three advantages:
> > > >
> > > > - no weird hookery needed
> > > > - resolution can go far beyond HZ
> > > > - it is evidently dynticks-safe
> > >
> > > Hmm, if we replace the HZ based oprofile tick with an hrtimer we
> > > should add an interface to configure the sample interval as well,
> > > no? Otherwise we just replace one timer event (HZ) with another
> > > (hrtimer).
> >
> > Even if the hrtimer is set with a 1/HZ period it's a better
> > solution, as it's dynticks safe without invasive changes.
>
> Ok, but the patch will be quite big. All the profile_tick() calls
> from the architecture files will have to be removed. [...]

Hey, that's a bonus :)

> [...] And if we really want to keep things separate there will be
> two sets of per-cpu hrtimer, one for the old style profiler and
> one for oprofile. Any preference for the user space interface to
> set the sample rate? A sysctl?

I dont think we want to keep things separate. Regarding old-style
profiler, does anyone still use it? There's now a superior in-tree
replacement for it, so we could phase it out.

A sysctl sounds like the most obvious way to set the sample period -
and it can default to 1/Hz.

Ingo