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Date: Mon, 26 Mar 2012 15:28:37 -0400
From: Rik van Riel <riel@redhat.com>
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To: Peter Zijlstra <a.p.zijlstra@chello.nl>
CC: 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>,
        Johannes Weiner <hannes@cmpxchg.org>
Subject: Re: [PATCH 11/39] autonuma: CPU follow memory algorithm
References: <1332783986-24195-1-git-send-email-aarcange@redhat.com>  <1332783986-24195-12-git-send-email-aarcange@redhat.com> <1332786353.16159.173.camel@twins>
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On 03/26/2012 02:25 PM, Peter Zijlstra wrote:
> On Mon, 2012-03-26 at 19:45 +0200, Andrea Arcangeli wrote:
>> @@ -3220,6 +3214,8 @@ need_resched:
>>
>>          post_schedule(rq);
>>
>> +       sched_autonuma_balance();
>> +
>>          sched_preempt_enable_no_resched();
>>          if (need_resched())
>>                  goto need_resched;
>
> I already told you, this isn't ever going to happen. You do _NOT_ put a
> for_each_online_cpu() loop in the middle of schedule().

Agreed, it looks O(N), but because every CPU will be calling
it its behaviour will be O(N^2) and has the potential to
completely break systems with a large number of CPUs.

Finding a lower overhead way of doing the balancing does not
seem like an unsurmountable problem.

> You also do not call stop_one_cpu(migration_cpu_stop) in schedule to
> force migrate the task you just scheduled to away from this cpu. That's
> retarded.
>
> Nacked-by: Peter Zijlstra<a.p.zijlstra@chello.nl>


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