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

[Paul E. McKenney]

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 :/
> 
> > 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..

Well, you -could- force a very low priority CPU-bound task to run on
all nohz_full CPUs.  Not necessarily a good idea, but a relatively
non-intrusive response to that particular error condition.

							Thanx, Paul

> Arguably we should issue a boot time warning if NOHZ_FULL is configured
> and the TSC watchdog is running.
> 
> > 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.
>