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

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Message-ID: <f8d72e47-7e84-cbd0-869f-69bf452a8bfb@mellanox.com>
Date:	Wed, 27 Jul 2016 10:12:02 -0400
From:	Chris Metcalf <cmetcalf@...lanox.com>
To:	Christoph Lameter <cl@...ux.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 7/27/2016 9:55 AM, Christoph Lameter wrote:
> 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).

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?

Thanks for your testing!

-- 
Chris Metcalf, Mellanox Technologies
http://www.mellanox.com