linux-kernel - Re: [RFC] Restrict kernel spawning of threads to a specified set of cpus.

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Message-ID: <CAOtvUMfhLQYK95uPm8idG_vYv-9yG2-KSkScjMBCSuvtffW1Dg@mail.gmail.com>
Date:	Tue, 10 Sep 2013 09:59:55 +0300
From:	Gilad Ben-Yossef <gilad@...yossef.com>
To:	Mike Galbraith <bitbucket@...ine.de>
Cc:	Christoph Lameter <cl@...ux.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Mike Frysinger <vapier@...too.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [RFC] Restrict kernel spawning of threads to a specified set of cpus.

Hi,


On Tue, Sep 10, 2013 at 9:47 AM, Mike Galbraith <bitbucket@...ine.de> wrote:
>
> On Tue, 2013-09-10 at 09:05 +0300, Gilad Ben-Yossef wrote:
> > Hi,
> >
> > On Thu, Sep 5, 2013 at 11:07 PM, Christoph Lameter <cl@...ux.com> wrote:
> > > I am not sure how to call this kernel option but we need something like
> > > that. I see drivers and the kernel spawning processes on the nohz cores.
> > > The name kthread is not really catching the purpose.
> > >
> > > os_cpus=? highlatency_cpus=?
> > >
> >
> > First off, thank you for doing this. It is very useful :-)
> >
> > Currently if one wishes to run a single task on an isolated CPU with
> > as little interference as possible, one needs to pass
> > rcu_nocbs, isolcpus, nohz_full parameters and now kthread parameter,
> > all pretty much with the same values
> >
> > I know some people won't like this, but can we perhaps fold all these
> > into a single parameter, perhaps even the existing isolcpus?
>
> isolcpus is supposed to go away, as cpusets can isolate CPUs, and can
> turn off load balancing.
>

And I'm all for that. I think cpusets is a much more elegant solution.

But... AFAIK currently cpusets cannot migrate timers that were registered on
a cpu prior to it being isolated via cpuset, designate RCU off loaded CPUs or
sets cpus as full nohz capable, or - it seems from this patch, keep off certain
kernel thread off a cpu.

This is no fault of cpusets, but it still means there are work loads
that it can't
support at this time.

So long as we must  have a kernel boot option, I prefer to have one
and not four of
them. Think of it this way - when we put all these capabilities into
cpusets, we'll have
just one kernel option to kill and not four.

Does that makes sense?

Gilad

>
> -Mike
>



-- 
Gilad Ben-Yossef
Chief Coffee Drinker
gilad@...yossef.com
Israel Cell: +972-52-8260388
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