linux-kernel - Re: [PATCH] sched/topology: Use Identity node only if required

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Date:   Wed, 8 Aug 2018 11:30:16 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
Cc:     Ingo Molnar <mingo@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Rik van Riel <riel@...riel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Michael Ellerman <mpe@...erman.id.au>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        Suravee Suthikulpanit <suravee.suthikulpanit@....com>,
        Andre Wild <wild@...ux.vnet.ibm.com>
Subject: Re: [PATCH] sched/topology: Use Identity node only if required

On Wed, Aug 08, 2018 at 10:43:02AM +0200, Peter Zijlstra wrote:
> On Wed, Aug 08, 2018 at 01:19:42AM -0700, Srikar Dronamraju wrote:

> > We could fix the numa topology to be NUMA_DIRECT for 2 node machines, by
> > checking if sched_domains_numa_levels == 2, but then I dont know what it
> > means for a system that has only NODE but not NUMA level.
> 
> You have a point there; I think we should not have added the NODE thing
> to sched_domains_numa_level, let me see if we can fix that sanely.

Hurm,. looking this over I don't see anything better than changing that
NUMA_DIRECT test to <= 2.

But as the comment says, DIRECT means: all nodes are directly connected,
or not a NUMA system. A system with a single node is not a NUMA system
and would thus qualify.