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

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Message-ID: <alpine.DEB.2.20.1608111801320.8416@east.gentwo.org>
Date:	Thu, 11 Aug 2016 18:02:34 -0500 (CDT)
From:	Christoph Lameter <cl@...ux.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
cc:	Peter Zijlstra <peterz@...radead.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Chris Metcalf <cmetcalf@...lanox.com>,
	Gilad Ben Yossef <giladb@...lanox.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Rik van Riel <riel@...hat.com>, Tejun Heo <tj@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	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 Thu, 11 Aug 2016, Paul E. McKenney wrote:

> > 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.

Given that we want the cpu only to run the user task I would think that is
not a good idea.