linux-kernel - Re: [RFC] sched: unused cpu in affine workload

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Date:	Mon, 4 Apr 2016 11:38:44 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Jiri Olsa <jolsa@...hat.com>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	James Hartsock <hartsjc@...hat.com>,
	Rik van Riel <riel@...hat.com>,
	Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>,
	Kirill Tkhai <ktkhai@...allels.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC] sched: unused cpu in affine workload

* Ingo Molnar <mingo@...nel.org> wrote:

> So my thinking here is: if the NUMA balancing code (which is node granular at 
> the moment and uses node masks, etc.) is extended to be CPU granular (which is a 
> big task in itself), then the two problems can be 'unified':
> 
>   - the NUMA balancing code inputs arbitrarly CPU (node) affinity masks from the
>     MM code into the scheduler.
> 
>   - the scheduler syscall ABI (and other configuration sources) inputs arbitrary 
>     CPU affinity masks into the scheduler.
> 
> it's a similar problem, with two (minor looking) complication:

btw., this highlights how hard the optimization problem is: the NUMA balancing 
code is (at least ...) O(nr_nodes^2) complex - but we had O(nr_nodes^3) passes too 
in some of the NUMA balancing submissions...

We'd upgrade that to O(nr_cpus^2), which is totally unrealistic with 16,000 CPUs 
even in a slowpath - but it would probably cause problems even with 120 CPUs. It 
will get quadratically worse as the number of CPUs in a system increases on its 
current exponential trajectory ...

So the safest bet would be to restrict any 'perfect' balancing attempts to node 
boundaries. Which won't solve the problem you outlined to begin with.

Thanks,

	Ingo