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

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Date:	Tue, 10 Sep 2013 09:26:35 +0200
From:	Mike Galbraith <bitbucket@...ine.de>
To:	Gilad Ben-Yossef <gilad@...yossef.com>
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.

On Tue, 2013-09-10 at 09:59 +0300, Gilad Ben-Yossef wrote: 
> 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?

Hammering on the wrong spot makes removing isolcpus take longer, and
adds up to more hammering in the long run, no?  Hearing you mention
isolcpus, I just thought I should mention that it wants to go away, so
might not be the optimal spot for isolation related tinkering.

-Mike

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