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Date:   Fri, 21 May 2021 10:14:10 +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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On Fri, May 21, 2021 at 08:08:02AM +0530, Srikar Dronamraju wrote:
> * Peter Zijlstra <peterz@infradead.org> [2021-05-20 20:56:31]:
> 
> > 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.
> 
> Since for an offline node, arch interface code doesn't have the info.
> As far as I know/understand, in POWER, unless there is an active memory or
> CPU that's getting onlined, arch can't fetch the correct node distance.
> 
> Taking the above example: node 3 is offline, then node_distance of (3,X)
> where X is anything other than 3, is not reliable. The moment node 3 is
> onlined, the node distance is reliable.
> 
> This problem will not happen even on POWER if all the nodes have either
> memory or CPUs active at the time of boot.

But then how can you implement this new hook? Going by the fact that
both nr_node_ids and distance_ref_points_depth are fixed, how many
possible __node_distance() configurations are there left?

The example provided above does not suggest there's much room for
alternatives, and hence for actual need of this new interface.