linux-kernel - Re: clocksource_watchdog causing scheduling of timers every second (was [v13] support "task

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Date:	Wed, 27 Jul 2016 10:23:12 -0500 (CDT)
From:	Christoph Lameter <cl@...ux.com>
To:	Chris Metcalf <cmetcalf@...lanox.com>
cc:	Gilad Ben Yossef <giladb@...lanox.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Rik van Riel <riel@...hat.com>, Tejun Heo <tj@...nel.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Viresh Kumar <viresh.kumar@...aro.org>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will.deacon@....com>,
	Andy Lutomirski <luto@...capital.net>,
	Daniel Lezcano <daniel.lezcano@...aro.org>,
	linux-doc@...r.kernel.org, linux-api@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: clocksource_watchdog causing scheduling of timers every second
 (was [v13] support "task_isolation" mode)

On Wed, 27 Jul 2016, Chris Metcalf wrote:

> > 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).
>
> That sounds like the right idea - use the housekeeping cpu mask instead of the
> cpu online mask.  Should be a straightforward patch; do you want to do that
> and test it in your configuration, and I'll include it in the next spin of the
> patch series?

Sadly housekeeping_mask is defined the following way:

static inline const struct cpumask *housekeeping_cpumask(void)
{
#ifdef CONFIG_NO_HZ_FULL
        if (tick_nohz_full_enabled())
                return housekeeping_mask;
#endif
        return cpu_possible_mask;
}

Why is it not returning cpu_online_mask?