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

Martin Schwidefsky
2009-06-22T15:36:22Z

On Mon, 22 Jun 2009 17:29:37 +0200
Ingo Molnar <mingo@elte.hu> wrote:

>
> * 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 :)

It would remove some oddball code :-)

> > [...] 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.

Well, for my part I won't miss it. But to be able to remove the
profile_tick() calls from the architectures I either have to rip out
the old profiler now, or adapt it to use hrtimer as well.

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

Agreed.