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SubjectRe: [PATCH 1/2] nohz: Disable LOCKUP_DETECTOR when NO_HZ_FULL is enabled
On Wed, May 15, 2013 at 10:37:29AM +0200, Peter Zijlstra wrote:
> On Tue, May 14, 2013 at 06:02:51PM +0200, Frederic Weisbecker wrote:
> > From: Steven Rostedt <rostedt@goodmis.org>
> >
> > Trying to test the nohz_full code, I was not able to get it to work.
> > Finally I enabled the tick_stop tracepoint and it showed:
> >
> > tick_stop: success=no msg=perf events running
> >
> > I talked to Frederic Weisbecker about this and he informed me that
> > perf is used by the lockup detector. I checked the code, and sure
> > enough it is.
> >
> > As perf is always running when LOCKUP_DETECTOR is enabled, which
> > will always disable nohz_full from working, instead of confusing
> > users, disable LOCKUP_DETECTOR when NO_HZ_FULL is enabled.
> >
> > When perf is changed such that it does not prevent nohz_full from
> > working, then we can and should remove this constraint.
>
> That's a bit contradictory in function, you want the NMI watchdog to
> cover all code, so disabling whilst entering NO_HZ state is going to
> make it not cover some code - *fail*.
>
> Rather I would suggest disabling the NMI watchdog's runtime default; so
> you can still enable it with something like:
>
> echo 1 > /proc/sys/kernel/nmi_watchdog
>

Sounds good, and we then warn the user about that.


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