linux-kernel - Re: [PATCH 1/3] sched/topology: Allow archs to populate distance map

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Message-ID: <87h7irglm9.mognet@arm.com>
Date:   Tue, 25 May 2021 11:21:02 +0100
From:   Valentin Schneider <valentin.schneider@....com>
To:     Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        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>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        linuxppc-dev@...ts.ozlabs.org,
        Nathan Lynch <nathanl@...ux.ibm.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Scott Cheloha <cheloha@...ux.ibm.com>,
        Gautham R Shenoy <ego@...ux.vnet.ibm.com>,
        Geetika Moolchandani <Geetika.Moolchandani1@....com>
Subject: Re: [PATCH 1/3] sched/topology: Allow archs to populate distance map

On 24/05/21 21:48, Srikar Dronamraju wrote:
> * Valentin Schneider <valentin.schneider@....com> [2021-05-24 15:16:09]:
>> Ok so from your arch you can figure out the *size* of the set of unique
>> distances, but not the individual node_distance(a, b)... That's quite
>> unfortunate.
>
> Yes, thats true.
>
>>
>> I suppose one way to avoid the hook would be to write some "fake" distance
>> values into your distance_lookup_table[] for offline nodes using your
>> distance_ref_point_depth thing, i.e. ensure an iteration of
>> node_distance(a, b) covers all distance values [1]. You can then keep patch
>> 3 around, and that should roughly be it.
>>
>
> Yes, this would suffice but to me its not very clean.
> static int found[distance_ref_point_depth];
>
> for_each_node(node){
>       int i, nd, distance = LOCAL_DISTANCE;
>               goto out;
>
>       nd = node_distance(node, first_online_node)
>       for (i=0; i < distance_ref_point_depth; i++, distance *= 2) {
>               if (node_online) {
>                       if (distance != nd)
>                               continue;
>                       found[i] ++;
>                       break;
>               }
>               if (found[i])
>                       continue;
>               distance_lookup_table[node][i] = distance_lookup_table[first_online_node][i];
>               found[i] ++;
>               break;
>       }
> }
>
> But do note: We are setting a precedent for node distance between two nodes
> to change.
>

Indeed. AFAICT it's that or the unique-distance-values hook :/