linux-kernel - Re: [PATCH 2/3] powerpc/numa: Populate distance map correctly

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Date:   Mon, 24 May 2021 20:20:40 +0530
From:   Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
To:     Valentin Schneider <valentin.schneider@....com>
Cc:     Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.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 2/3] powerpc/numa: Populate distance map correctly

* Valentin Schneider <valentin.schneider@....com> [2021-05-24 15:16:22]:

> On 20/05/21 21:14, Srikar Dronamraju wrote:
> > +int arch_populate_distance_map(unsigned long *distance_map)
> > +{
> > +	int i;
> > +	int distance = LOCAL_DISTANCE;
> > +
> > +	bitmap_set(distance_map, distance, 1);
> > +
> > +	if (!form1_affinity) {
> > +		bitmap_set(distance_map, REMOTE_DISTANCE, 1);
> > +		return 0;
> > +	}
> > +
> > +	for (i = 0; i < distance_ref_points_depth; i++) {
> > +		distance *= 2;
> > +		bitmap_set(distance_map, distance, 1);
> 
> Do you have guarantees your distance values will always be in the form of
> 
>   LOCAL_DISTANCE * 2^i
> 
> because that certainly isn't true for x86/arm64.
> 

This is true till now. It don't think that's going to change anytime soon, but
we never know what lies ahead.

For all practical purposes, (unless a newer, shinier property is proposed,)
distance_ref_points_depth is going to give us the unique distances.

> > +	}
> > +	return 0;
> > +}
> > +
> >  /*
> >   * Returns nid in the range [0..nr_node_ids], or -1 if no useful NUMA
> >   * info is found.
> > --
> > 2.27.0

-- 
Thanks and Regards
Srikar Dronamraju