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Date:   Thu, 20 May 2021 20:56:31 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Srikar Dronamraju <srikar@linux.vnet.ibm.com>
Cc:     Ingo Molnar <mingo@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Mel Gorman <mgorman@techsingularity.net>,
        Rik van Riel <riel@surriel.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Valentin Schneider <valentin.schneider@arm.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        linuxppc-dev@lists.ozlabs.org,
        Nathan Lynch <nathanl@linux.ibm.com>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Scott Cheloha <cheloha@linux.ibm.com>,
        Gautham R Shenoy <ego@linux.vnet.ibm.com>,
        Geetika Moolchandani <Geetika.Moolchandani1@ibm.com>
Subject: Re: [PATCH 1/3] sched/topology: Allow archs to populate distance map
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References: <20210520154427.1041031-1-srikar@linux.vnet.ibm.com>
 <20210520154427.1041031-2-srikar@linux.vnet.ibm.com>
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On Thu, May 20, 2021 at 09:14:25PM +0530, Srikar Dronamraju wrote:
> Currently scheduler populates the distance map by looking at distance
> of each node from all other nodes. This should work for most
> architectures and platforms.
> 
> However there are some architectures like POWER that may not expose
> the distance of nodes that are not yet onlined because those resources
> are not yet allocated to the OS instance. Such architectures have
> other means to provide valid distance data for the current platform.
> 
> For example distance info from numactl from a fully populated 8 node
> system at boot may look like this.
> 
> node distances:
> node   0   1   2   3   4   5   6   7
>   0:  10  20  40  40  40  40  40  40
>   1:  20  10  40  40  40  40  40  40
>   2:  40  40  10  20  40  40  40  40
>   3:  40  40  20  10  40  40  40  40
>   4:  40  40  40  40  10  20  40  40
>   5:  40  40  40  40  20  10  40  40
>   6:  40  40  40  40  40  40  10  20
>   7:  40  40  40  40  40  40  20  10
> 
> However the same system when only two nodes are online at boot, then the
> numa topology will look like
> node distances:
> node   0   1
>   0:  10  20
>   1:  20  10
> 
> It may be implementation dependent on what node_distance(0,3) where
> node 0 is online and node 3 is offline. In POWER case, it returns
> LOCAL_DISTANCE(10). Here at boot the scheduler would assume that the max
> distance between nodes is 20. However that would not be true.
> 
> When Nodes are onlined and CPUs from those nodes are hotplugged,
> the max node distance would be 40.
> 
> To handle such scenarios, let scheduler allow architectures to populate
> the distance map. Architectures that like to populate the distance map
> can overload arch_populate_distance_map().

Why? Why can't your node_distance() DTRT? The arch interface is
nr_node_ids and node_distance(), I don't see why we need something new
and then replace one special use of it.

By virtue of you being able to actually implement this new hook, you
supposedly can actually do node_distance() right too.