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

[Valentin Schneider <valentin.schneider@arm.com>]

On 21/05/21 14:58, Srikar Dronamraju wrote:
> * Peter Zijlstra <peterz@infradead.org> [2021-05-21 10:14:10]:
>
>> 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?
>>
>
> distance_ref_point_depth is provided as a different property and is readily
> available at boot. The new api will use just use that. So based on the
> distance_ref_point_depth, we know all possible node distances for that
> platform.
>
> For an offline node, we don't have that specific nodes distance_lookup_table
> array entries. Each array would be of distance_ref_point_depth entries.
> Without the distance_lookup_table for an array populated, we will not be
> able to tell how far the node is with respect to other nodes.
>
> We can lookup the correct distance_lookup_table for a node based on memory
> or the CPUs attached to that node. Since in an offline node, both of them
> would not be around, the distance_lookup_table will have stale values.
>

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.

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.


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