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Message-Id: <20160811160027.GV3482@linux.vnet.ibm.com>
Date: Thu, 11 Aug 2016 09:00:27 -0700
From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Frederic Weisbecker <fweisbec@...il.com>,
Christoph Lameter <cl@...ux.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, Aug 11, 2016 at 10:40:02AM +0200, Peter Zijlstra wrote:
> On Thu, Aug 11, 2016 at 12:16:58AM +0200, Frederic Weisbecker wrote:
> > I had similar issues, this seems to happen when the tsc is considered not reliable
> > (which doesn't necessarily mean unstable. I think it has to do with some x86 CPU feature
> > flag).
>
> Right, as per the other email, in general we cannot know/assume the TSC
> to be working as intended :/
>
> > IIRC, this _has_ to execute on all online CPUs because every TSCs of running CPUs
> > are concerned.
>
> 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.
Thanx, Paul
> Arguably we should issue a boot time warning if NOHZ_FULL is configured
> and the TSC watchdog is running.
>
> > I personally override that with passing the tsc=reliable kernel
> > parameter. Of course use it at your own risk.
>
> Yes, that is (sadly) our only option. Manually assert our hardware is
> solid under the intended workload and then manually disabling the
> watchdog.
>
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