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

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Message-ID: <20210528102434.GO2633526@linux.vnet.ibm.com>
Date:   Fri, 28 May 2021 15:54:34 +0530
From:   Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Valentin Schneider <valentin.schneider@....com>,
        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

* Peter Zijlstra <peterz@...radead.org> [2021-05-28 10:43:23]:

> On Mon, May 24, 2021 at 09:48:29PM +0530, Srikar Dronamraju wrote:
> > * Valentin Schneider <valentin.schneider@....com> [2021-05-24 15:16:09]:
> 
> > > 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.
> 
> Not really; or rather not more than already is the case AFAICT. Because
> currently your distance table will have *something* in it
> (LOCAL_DISTANCE afaict) for nodes that have never been online, which is
> what triggered the whole problem to begin with.
> 
> Only after the node has come online for the first time, will it contain
> the right value.
> 
> So both before and after this proposal the actual distance value changes
> after the first time a node goes online.
> 
> Yes that's unfortunate, but I don't see a problem with pre-filling it
> with something useful in order to avoid aditional arch hooks.
> 
> 

Okay,

Will post a v2 with prefilling.
Thanks for the update.

-- 
Thanks and Regards
Srikar Dronamraju