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

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Date:   Fri, 28 May 2021 10:51:03 +0530
From:   Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
To:     Valentin Schneider <valentin.schneider@....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

* Valentin Schneider <valentin.schneider@....com> [2021-05-25 11:21:02]:

> 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 :/

Peter, Valentin, Michael,

Can you please let me know which approach you would want me to follow.

Or do let me know any other alternative solutions that you would want me to
try.


-- 
Thanks and Regards
Srikar Dronamraju