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

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Date:   Wed, 8 Aug 2018 10:43:02 +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 01:19:42AM -0700, Srikar Dronamraju wrote:

> However, I still think if majority of the cases the identity node is
> going to be redundant, then we should use hint.

No, same way we always generate SMT domains when we have
CONFIG_SCHED_SMT irrespective of the hardware having SMT.

Also, an arch hook like you did is just fugly. If anything you add the
single node thing to the regular topology setup of x86_numa_in_package.

But the thing is, even for x86_numa_in_package the single node domain is
mostly redundant because the MC domain will match the NODE domain most
times.

> 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.