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

[Frederic Weisbecker <fweisbec@gmail.com>]

On Wed, Jul 27, 2016 at 08:55:28AM -0500, Christoph Lameter wrote:
> On Mon, 25 Jul 2016, Christoph Lameter wrote:
> 
> > Guess so. I will have a look at this when I get some time again.
> 
> Ok so the problem is the clocksource_watchdog() function in
> kernel/time/clocksource.c. This function is active if
> CONFIG_CLOCKSOURCE_WATCHDOG is defined. It will check the timesources of
> each processor for being within bounds and then reschedule itself on the
> next one.
> 
> The purpose of the function seems to be to determine *if* a clocksource is
> unstable. It does not mean that the clocksource *is* unstable.
> 
> The critical piece of code is this:
> 
>         /*
>          * Cycle through CPUs to check if the CPUs stay synchronized
>          * to each other.
>          */
>         next_cpu = cpumask_next(raw_smp_processor_id(), cpu_online_mask);
>         if (next_cpu >= nr_cpu_ids)
>                 next_cpu = cpumask_first(cpu_online_mask);
>         watchdog_timer.expires += WATCHDOG_INTERVAL;
>         add_timer_on(&watchdog_timer, next_cpu);
> 
> 
> Should we just cycle through the cpus that are not isolated? Otherwise we
> need to have some means to check the clocksources for accuracy remotely
> (probably impossible for TSC etc).
> 
> The WATCHDOG_INTERVAL is 1 second so this causes an interrupt every
> second.
> 
> Note that we are running with the patch that removes the 1 HZ mininum time
> tick. With an older kernel code base (redhat) we can keep the kernel quiet
> for minutes. The clocksource watchdog causes timers to fire again.

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).

IIRC, this _has_ to execute on all online CPUs because every TSCs of running CPUs
are concerned.

I personally override that with passing the tsc=reliable kernel parameter. Of course
use it at your own risk.

But eventually I don't think we can offline that to housekeeping only CPUs.