Re: clocksource_watchdog causing scheduling of timers every second (was [v13] support "task_isolation" mode)

[Frederic Weisbecker <fweisbec@gmail.com>]

On Thu, Aug 11, 2016 at 10:40:02AM +0200, Peter Zijlstra wrote:
> On Thu, Aug 11, 2016 at 12:16:58AM +0200, Frederic Weisbecker wrote:
> > I had similar issues, this seems to happen when the tsc is considered not reliable
> > (which doesn't necessarily mean unstable. I think it has to do with some x86 CPU feature
> > flag).
> 
> Right, as per the other email, in general we cannot know/assume the TSC
> to be working as intended :/

Yeah, I remember you explained me that a little while ago.

> 
> > IIRC, this _has_ to execute on all online CPUs because every TSCs of running CPUs
> > are concerned.
> 
> With modern Intel we could run it on one CPU per package I think, but at
> the same time, too much in NOHZ_FULL assumes the TSC is indeed sane so
> it doesn't make sense to me to keep the watchdog running, when it
> triggers it would also have to kill all NOHZ_FULL stuff, which would
> probably bring the entire machine down..
> 
> Arguably we should issue a boot time warning if NOHZ_FULL is configured
> and the TSC watchdog is running.

That's a very good idea! We do that when tsc is unstable but indeed we can't
seriously run NOHZ_FULL on a non-reliable tsc.

I'll take care of that warning.

> 
> > I personally override that with passing the tsc=reliable kernel
> > parameter. Of course use it at your own risk.
> 
> Yes, that is (sadly) our only option. Manually assert our hardware is
> solid under the intended workload and then manually disabling the
> watchdog.

Right, I'll tell about that in the warning.

Thanks for those details!