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

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Date:	Wed, 27 Jul 2016 15:58:14 -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 3:53 PM, Christoph Lameter wrote:
> On Wed, 27 Jul 2016, Chris Metcalf wrote:
>
>> Looks good.  Did you omit the equivalent fix in clocksource_start_watchdog()
>> on purpose?  For now I just took your change, but tweaked it to add the
>> equivalent diff with cpumask_first_and() there.
> Can the watchdog be started on an isolated cpu at all? I would expect that
> the code would start a watchdog only on a housekeeping cpu.

The code just starts the watchdog initially on the first online cpu.
In principle you could have configured that as an isolated cpu, so
without any change to that code, you'd interrupt that cpu.

I guess another way to slice it would be to start the watchdog on the
current core.  But just using the same idiom as in clocksource_watchdog()
seems cleanest to me.

I added your patch to the series and pushed it up (along with adding your
Tested-by to the x86 enablement commit).  It's still based on 4.6 so I'll need
to rebase it once the merge window closes.

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