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

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Date:	Fri, 12 Aug 2016 09:19:19 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	Christoph Lameter <cl@...ux.com>,
	Peter Zijlstra <peterz@...radead.org>,
	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 Fri, Aug 12, 2016 at 04:26:13PM +0200, Frederic Weisbecker wrote:
> On Fri, Aug 12, 2016 at 09:23:13AM -0500, Christoph Lameter wrote:
> > On Thu, 11 Aug 2016, Paul E. McKenney wrote:
> > 
> > > Heh!  The only really good idea is for clocks to be reliably in sync.
> > >
> > > But if they go out of sync, what do you want to do instead?
> > 
> > For a NOHZ task? Write a message to the syslog and reenable tick.

Fair enough!  Kicking off a low-priority task would achieve the latter
but not necessarily the former.  And of course assumes that the worker
thread is at real-time priority with various scheduler anti-starvation
features disabled.

> Indeed, a strong clocksource is a requirement for a full tickless machine.

No disagrement here!  ;-)

							Thanx, Paul