linux-kernel - Re: [PATCH 07/39] autonuma: introduce kthread_bind

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Date:	Tue, 27 Mar 2012 18:19:52 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Andrea Arcangeli <aarcange@...hat.com>
Cc:	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Hillf Danton <dhillf@...il.com>, Dan Smith <danms@...ibm.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>, Paul Turner <pjt@...gle.com>,
	Suresh Siddha <suresh.b.siddha@...el.com>,
	Mike Galbraith <efault@....de>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	Bharata B Rao <bharata.rao@...il.com>,
	Lee Schermerhorn <Lee.Schermerhorn@...com>,
	Rik van Riel <riel@...hat.com>,
	Johannes Weiner <hannes@...xchg.org>
Subject: Re: [PATCH 07/39] autonuma: introduce kthread_bind_node()

On Tue, 2012-03-27 at 18:04 +0200, Andrea Arcangeli wrote:
> On Tue, Mar 27, 2012 at 05:45:35PM +0200, Peter Zijlstra wrote:
> > On Tue, 2012-03-27 at 17:22 +0200, Andrea Arcangeli wrote:
> > > I don't see what's wrong with more than 1 CPU in the hard bind
> > > cpumask.
> > 
> > Because its currently broken, but we're trying to restore its pure
> > semantic so that we can use it in more places again, like
> > debug_smp_processor_id(). Testing a single process flag is _much_
> > cheaper than testing ->cpus_allowed.
> > 
> > Adding more broken isn't an option.
> 
> I would suggest you to use a new bitflag for that _future_
> optimization that you plan to do without altering the way the current
> bitflag works.
> 
> I doubt knuma_migrated will ever be the only kernel thread that wants
> to run with a NUMA NODE-wide CPU binding (instead of single-CPU
> binding).
> 
> Being able to keep using this bitflag for NUMA-wide bindings too in
> the future as well (after you do the optimization you planned), is
> going to reduce the chances of the root user shooting himself in the
> foot for both the kernel thread node-BIND and the single-cpu-BIND.

But then the current flag is a mis-nomer. Also, there's no correctness
issue with the per-node threads, its perfectly fine if they run some
place else so I don't think we should restrict userspace to force them
away from their preferred node.

So even if you were to introduce a new flag, I'd still object.

The only reason to ever refuse userspace moving a task around is if it
will break stuff. Worst that can happen with a node affine thread is
that it'll incur remote memory penalties, that's not fatal.
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