Re: [RFC -v3 2/2] watchdog: update watchdog_tresh properly

[Don Zickus <dzickus@redhat.com>]

On Tue, Jul 23, 2013 at 04:07:29PM +0200, Michal Hocko wrote:
> On Tue 23-07-13 09:53:34, Don Zickus wrote:
> > On Mon, Jul 22, 2013 at 04:32:46PM +0200, Michal Hocko wrote:
> > > The nmi one is disabled and then reinitialized from scratch. This
> > > has an unpleasant side effect that the allocation of the new event might
> > > fail theoretically so the hard lockup detector would be disabled for
> > > such cpus. On the other hand such a memory allocation failure is very
> > > unlikely because the original event is deallocated right before.
> > > It would be much nicer if we just changed perf event period but there
> > > doesn't seem to be any API to do that right now.
> > > It is also unfortunate that perf_event_alloc uses GFP_KERNEL allocation
> > > unconditionally so we cannot use on_each_cpu() and do the same thing
> > > from the per-cpu context. The update from the current CPU should be
> > > safe because perf_event_disable removes the event atomically before
> > > it clears the per-cpu watchdog_ev so it cannot change anything under
> > > running handler feet.
> > 
> > I guess I don't have a problem with this.  I was hoping to have more
> > shared code with the regular stop/start routines but with the pmu bit
> > locking (to share pmus with oprofile), you really need to unregister
> > everything to stop the lockup detector.  This makes it a little too heavy
> > for a restart routine like this.
> 
> I am not sure I understand the above. Regular stop/start is about all
> the machinery, I have tried to reduce the restarting to bare minimum.
> Do you find the current version heavier than the full disable_all &&
> enable_all?

No, I find your restart mechanism lighter than full disable_all.  I would
love to have the lockup detector just disable itself on stop and re-enable
on start.  But because of oprofile, the lockup has to free up its event
on stop and recreate it on start, which kinda sucks.

Anyway it was just an aside.

> 
> > The only odd thing is I can't figure out which version you were using to
> > apply this patch.  I can't find old_thresh (though I understand the idea
> > of it).
> 
> current Linus tree (linux-next - 20130723 - has it as well AFAICS)

Ok. Thanks.  Ah, I see.  I forgot Frederic modified pieces there.  The
threading keeps changing.  I see why you took your approach.

Should be fine.

Acked-by: Don Zickus <dzickus@redhat.com>