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

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Message-ID: <20140123135430.GB13345@localhost.localdomain>
Date:	Thu, 23 Jan 2014 14:54:32 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Lei Wen <adrian.wenl@...il.com>
Cc:	Viresh Kumar <viresh.kumar@...aro.org>,
	Kevin Hilman <khilman@...aro.org>,
	Lists linaro-kernel <linaro-kernel@...ts.linaro.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Linaro Networking <linaro-networking@...aro.org>,
	Tejun Heo <tj@...nel.org>
Subject: Re: [QUERY]: Is using CPU hotplug right for isolating CPUs?

On Tue, Jan 21, 2014 at 10:07:58AM +0800, Lei Wen wrote:
> On Mon, Jan 20, 2014 at 11:41 PM, Frederic Weisbecker
> <fweisbec@...il.com> wrote:
> > On Mon, Jan 20, 2014 at 08:30:10PM +0530, Viresh Kumar wrote:
> >> On 20 January 2014 19:29, Lei Wen <adrian.wenl@...il.com> wrote:
> >> > Hi Viresh,
> >>
> >> Hi Lei,
> >>
> >> > I have one question regarding unbounded workqueue migration in your case.
> >> > You use hotplug to migrate the unbounded work to other cpus, but its cpu mask
> >> > would still be 0xf, since cannot be changed by cpuset.
> >> >
> >> > My question is how you could prevent this unbounded work migrate back
> >> > to your isolated cpu?
> >> > Seems to me there is no such mechanism in kernel, am I understand wrong?
> >>
> >> These workqueues are normally queued back from workqueue handler. And we
> >> normally queue them on the local cpu, that's the default behavior of workqueue
> >> subsystem. And so they land up on the same CPU again and again.
> >
> > But for workqueues having a global affinity, I think they can be rescheduled later
> > on the old CPUs. Although I'm not sure about that, I'm Cc'ing Tejun.
> 
> Agree, since worker thread is made as enterring into all cpus, it
> cannot prevent scheduler
> do the migration.
> 
> But here is one point, that I see Viresh alredy set up two cpuset with
> scheduler load balance
> disabled, so it should stop the task migration between those two groups? Since
> the sched_domain changed?
> 
> What is more, I also did  similiar test, and find when I set two such
> cpuset group,
> like core 0-2 to cpuset1, core 3 to cpuset2, while hotunplug the core3
> afterwise.
> I find the cpuset's cpus member becomes NULL even I hotplug the core3
> back again.
> So is it a bug?

Not sure, you may need to check cpuset internals.
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