Re: nohz: Warn if the machine can not perform nohz_full

From: Steven Rostedt
Date: Mon Jul 15 2013 - 14:49:48 EST


On Mon, 2013-07-15 at 13:38 -0400, Dave Jones wrote:
> On Mon, Jul 15, 2013 at 01:24:23PM -0400, Dave Jones wrote:
> > On Mon, Jul 15, 2013 at 07:18:02PM +0200, Frederic Weisbecker wrote:
> >
> > > > So I guess you guys never want this to be enabled on distro kernels ?
> > > > If that's the case, can you add something to that effect in Kconfig ?
> > >
> > > I believe we want it to be enabled on distros in the long term. But right now it would
> > > be a bad idea until the off case (nohz_full= parameter empty) is carefully optimized.
> > > I'm currently working on that.
> > >
> > > Now for the unstable tsc, which is what it's about on the above code block, we need
> > > the tick to be there to leverage the sched clock madness. May be there could be some
> > > other solution that could work along full dynticks but for now we chose the easy path.
> > >
> > > Are broken TSCs that common?
> >
> > I just hit one apparently. http://paste.fedoraproject.org/25421/73907845/raw/
> > That's a fairly recent Atom board, so I suspect it's not uncommon on that platform.
>
> And here's a Core Duo from circa 2008.
> http://paste.fedoraproject.org/25429/13739098/raw
>
> Two for two so far. I get the feeling you guys are going to get a ton of these reports.
>

Hmm, we should only warn if the user tried to enable nohz-full via the
command line. But it looks like it warns even without enabling
nohz-full, which wasn't the desired effect.

I'll look at this, and send a patch to make sure the warning only
happens when the user tries to use nohz-full, and doesn't just compile
it in. The point of the patch is to not let the user think they have
nohz-full when they don't.

-- Steve


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