linux-kernel - Re: NMI watchdog triggering during load

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Date:	Fri, 06 Mar 2015 08:10:57 -0700
From:	David Ahern <david.ahern@...cle.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	Mike Galbraith <efault@....de>, Ingo Molnar <mingo@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: NMI watchdog triggering during load_balance

On 3/6/15 2:07 AM, Peter Zijlstra wrote:
> On Thu, Mar 05, 2015 at 09:05:28PM -0700, David Ahern wrote:
>> Since each domain is a superset of the lower one each pass through
>> load_balance regularly repeats the processing of the previous domain (e.g.,
>> NODE domain repeats the cpus in the CPU domain). Then multiplying that
>> across 1024 cpus and it seems like a of duplication.
>
> It is, _but_ each domain has an interval, bigger domains _should_ load
> balance at a bigger interval (iow lower frequency), and all this is
> lockless data gathering, so reusing stuff from the previous round could
> be quite stale indeed.
>

Yes and I have twiddled the intervals. The defaults for min_interval and 
max_interval (msec):
SMT 1 2
MC  1 4
CPU 1 4
NODE 8 32

Increasing those values (e.g. moving NODE to 50 and 100) drops idle time 
cpu usage but does not solve the fundamental problem -- under load the 
balancing of domains seems to be lining up and the system comes to a 
halt in load balancing frenzy.

David
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