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

[Srikar Dronamraju]

* Peter Zijlstra <peterz@xxxxxxxxxxxxx> [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.

-- 
Thanks and Regards
Srikar Dronamraju