linux-kernel - Re: [QUERY]: Is using CPU hotplug right for isolating CPUs?

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Message-ID: <20140115103808.GY31570@twins.programming.kicks-ass.net>
Date:	Wed, 15 Jan 2014 11:38:08 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Viresh Kumar <viresh.kumar@...aro.org>
Cc:	Frédéric Weisbecker <fweisbec@...il.com>,
	Kevin Hilman <khilman@...aro.org>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Amit Kucheria <amit.kucheria@...aro.org>,
	Lists linaro-kernel <linaro-kernel@...ts.linaro.org>,
	Linaro Networking <linaro-networking@...aro.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [QUERY]: Is using CPU hotplug right for isolating CPUs?

On Wed, Jan 15, 2014 at 02:57:36PM +0530, Viresh Kumar wrote:
> Hi Again,
> 
> I am now successful in isolating a CPU completely using CPUsets,
> NO_HZ_FULL and CPU hotplug..
> 
> My setup and requirements for those who weren't following the
> earlier mails:
> 
> For networking machines it is required to run data plane threads on
> some CPUs (i.e. one thread per CPU) and these CPUs shouldn't be
> interrupted by kernel at all.
> 
> Earlier I tried CPUSets with NO_HZ by creating two groups with
> load_balancing disabled between them and manually tried to move
> all tasks out to CPU0 group. But even then there were interruptions
> which were continuously coming on CPU1 (which I am trying to
> isolate). These were some workqueue events, some timers (like
> prandom), timer overflow events (As NO_HZ_FULL pushes hrtimer
> to long ahead in future, 450 seconds, rather than disabling them
> completely, and these hardware timers were overflowing their
> counters after 90 seconds on Samsung Exynos board).
> 
> So after creating CPUsets I hotunplugged CPU1 and added it back
> immediately. This moved all these interruptions away and now
> CPU1 is running my single thread ("stress") for ever.
> 
> Now my question is: Is there anything particularly wrong about using
> hotplugging here ? Will that lead to a disaster :)

Nah, its just ugly and we should fix it. You need to be careful to not
place tasks in a cpuset you're going to unplug though, that'll give
funny results.


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