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

[Peter Zijlstra]

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..

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.