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Date: Thu, 5 Jul 2012 22:33:35 +0400
From: Glauber Costa <glommer@parallels.com>
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Subject: Re: [PATCH 09/40] autonuma: introduce kthread_bind_node()
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To: Johannes Weiner <hannes@cmpxchg.org>
Cc: Rik van Riel <riel@redhat.com>, Andrea Arcangeli <aarcange@redhat.com>, linux-kernel@vger.kernel.org, linux-mm@kvack.org, Hillf Danton <dhillf@gmail.com>, Dan Smith <danms@us.ibm.com>, Linus Torvalds <torvalds@linux-foundation.org>, Andrew Morton <akpm@linux-foundation.org>, Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@elte.hu>, Paul Turner <pjt@google.com>, Suresh Siddha <suresh.b.siddha@intel.com>, Mike Galbraith <efault@gmx.de>, "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>, Lai Jiangshan <laijs@cn.fujitsu.com>, Bharata B Rao <bharata.rao@gmail.com>, Lee Schermerhorn <Lee.Schermerhorn@hp.com>, Srivatsa Vaddagiri <vatsa@linux.vnet.ibm.com>, Christoph Lameter <cl@linux.com>, Alex Shi <alex.shi@intel.com>, Mauricio Faria de Oliveira <mauricfo@linux.vnet.ibm.com>, Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>, Don Morris <don.morris@hp.com>, Benjamin Herrenschmidt <benh@kernel.crashing.org>

On 07/05/2012 05:09 PM, Johannes Weiner wrote:
> In the very first review iteration of AutoNUMA, Peter argued that the
> scheduler people want to use this flag in other places where they rely
> on this thing meaning a single cpu, not a group of them (check out the
> cpumask test in debug_smp_processor_id() in lib/smp_processor_id.c).
> 
> He also argued that preventing root from rebinding the numa daemons is
> not critical to this feature at all.  And I have to agree.

Despite not being a scheduler expert, I'll have to side with that as
well. The thing I have in mind is: We have people whose usecase depend
on completely isolating cpus, with nothing but a specialized task
running on it. For those people, even the hard binding between cpu0 and
the timer interrupt is a big problem.

If you force a per-node binding of a kthread, you are basically saying
that those people are unable to isolate a node. Or else, that they have
to choose between that, and AutoNUMA. Both are suboptimal choices, to
say the least.

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