Re: [PATCH v2 1/2] sched/topology: Skip updating masks for non-online nodes

[Srikar Dronamraju <srikar@linux.vnet.ibm.com>]

* Valentin Schneider <valentin.schneider@arm.com> [2021-08-09 13:52:38]:

> On 09/08/21 12:22, Srikar Dronamraju wrote:
> > * Valentin Schneider <valentin.schneider@arm.com> [2021-08-08 16:56:47]:
> >> Wait, doesn't the distance matrix (without any offline node) say
> >>
> >>   distance(0, 3) == 40
> >>
> >> ? We should have at the very least:
> >>
> >>   node   0   1   2   3
> >>     0:  10  20  ??  40
> >>     1:  20  20  ??  40
> >>     2:  ??  ??  ??  ??
> >>     3:  40  40  ??  10
> >>
> >
> > Before onlining node 3 and CPU 3 (node/CPU 0 and 1 are already online)
> > Note: Node 2-7 and CPU 2-7 are still offline.
> >
> > node   0   1   2   3
> >   0:  10  20  40  10
> >   1:  20  20  40  10
> >   2:  40  40  10  10
> >   3:  10  10  10  10
> >
> > NODE->mask(0) == 0
> > NODE->mask(1) == 1
> > NODE->mask(2) == 0
> > NODE->mask(3) == 0
> >
> > Note: This is with updating Node 2's distance as 40 for figuring out
> > the number of numa levels. Since we have all possible distances, we
> > dont update Node 3 distance, so it will be as if its local to node 0.
> >
> > Now when Node 3 and CPU 3 are onlined
> > Note: Node 2, 3-7 and CPU 2, 3-7 are still offline.
> >
> > node   0   1   2   3
> >   0:  10  20  40  40
> >   1:  20  20  40  40
> >   2:  40  40  10  40
> >   3:  40  40  40  10
> >
> > NODE->mask(0) == 0
> > NODE->mask(1) == 1
> > NODE->mask(2) == 0
> > NODE->mask(3) == 0,3
> >
> > CPU 0 continues to be part of Node->mask(3) because when we online and
> > we find the right distance, there is no API to reset the numa mask of
> > 3 to remove CPU 0 from the numa masks.
> >
> > If we had an API to clear/set sched_domains_numa_masks[node][] when
> > the node state changes, we could probably plug-in to clear/set the
> > node masks whenever node state changes.
> >
> 
> Gotcha, this is now coming back to me...
> 
> [...]
> 
> >> Ok, so it looks like we really can't do without that part - even if we get
> >> "sensible" distance values for the online nodes, we can't divine values for
> >> the offline ones.
> >>
> >
> > Yes
> >
> 
> Argh, while your approach does take care of the masks, it leaves
> sched_numa_topology_type unchanged. You *can* force an update of it, but
> yuck :(
> 
> I got to the below...
> 

Yes, I completely missed that we should update sched_numa_topology_type.


> ---
> From: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
> Date: Thu, 1 Jul 2021 09:45:51 +0530
> Subject: [PATCH 1/1] sched/topology: Skip updating masks for non-online nodes
> 
> The scheduler currently expects NUMA node distances to be stable from init
> onwards, and as a consequence builds the related data structures
> once-and-for-all at init (see sched_init_numa()).
> 
> Unfortunately, on some architectures node distance is unreliable for
> offline nodes and may very well change upon onlining.
> 
> Skip over offline nodes during sched_init_numa(). Track nodes that have
> been onlined at least once, and trigger a build of a node's NUMA masks when
> it is first onlined post-init.
> 

Your version is much much better than mine.
And I have verified that it works as expected.


> Reported-by: Geetika Moolchandani <Geetika.Moolchandani1@ibm.com>
> Signed-off-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
> Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
> ---
>  kernel/sched/topology.c | 65 +++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 65 insertions(+)
> 
> diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
> index b77ad49dc14f..cba95793a9b7 100644
> --- a/kernel/sched/topology.c
> +++ b/kernel/sched/topology.c
> @@ -1482,6 +1482,8 @@ int				sched_max_numa_distance;
>  static int			*sched_domains_numa_distance;
>  static struct cpumask		***sched_domains_numa_masks;
>  int __read_mostly		node_reclaim_distance = RECLAIM_DISTANCE;
> +
> +static unsigned long __read_mostly *sched_numa_onlined_nodes;
>  #endif
> 
>  /*
> @@ -1833,6 +1835,16 @@ void sched_init_numa(void)
>  			sched_domains_numa_masks[i][j] = mask;
> 
>  			for_each_node(k) {
> +				/*
> +				 * Distance information can be unreliable for
> +				 * offline nodes, defer building the node
> +				 * masks to its bringup.
> +				 * This relies on all unique distance values
> +				 * still being visible at init time.
> +				 */
> +				if (!node_online(j))
> +					continue;
> +
>  				if (sched_debug() && (node_distance(j, k) != node_distance(k, j)))
>  					sched_numa_warn("Node-distance not symmetric");
> 
> @@ -1886,6 +1898,53 @@ void sched_init_numa(void)
>  	sched_max_numa_distance = sched_domains_numa_distance[nr_levels - 1];
> 
>  	init_numa_topology_type();
> +
> +	sched_numa_onlined_nodes = bitmap_alloc(nr_node_ids, GFP_KERNEL);
> +	if (!sched_numa_onlined_nodes)
> +		return;
> +
> +	bitmap_zero(sched_numa_onlined_nodes, nr_node_ids);
> +	for_each_online_node(i)
> +		bitmap_set(sched_numa_onlined_nodes, i, 1);
> +}
> +
> +void __sched_domains_numa_masks_set(unsigned int node)
> +{
> +	int i, j;
> +
> +	/*
> +	 * NUMA masks are not built for offline nodes in sched_init_numa().
> +	 * Thus, when a CPU of a never-onlined-before node gets plugged in,
> +	 * adding that new CPU to the right NUMA masks is not sufficient: the
> +	 * masks of that CPU's node must also be updated.
> +	 */
> +	if (test_bit(node, sched_numa_onlined_nodes))
> +		return;
> +
> +	bitmap_set(sched_numa_onlined_nodes, node, 1);
> +
> +	for (i = 0; i < sched_domains_numa_levels; i++) {
> +		for (j = 0; j < nr_node_ids; j++) {
> +			if (!node_online(j) || node == j)
> +				continue;
> +
> +			if (node_distance(j, node) > sched_domains_numa_distance[i])
> +				continue;
> +
> +			/* Add remote nodes in our masks */
> +			cpumask_or(sched_domains_numa_masks[i][node],
> +				   sched_domains_numa_masks[i][node],
> +				   sched_domains_numa_masks[0][j]);
> +		}
> +	}
> +
> +	/*
> +	 * A new node has been brought up, potentially changing the topology
> +	 * classification.
> +	 *
> +	 * Note that this is racy vs any use of sched_numa_topology_type :/
> +	 */
> +	init_numa_topology_type();
>  }
> 
>  void sched_domains_numa_masks_set(unsigned int cpu)
> @@ -1893,8 +1952,14 @@ void sched_domains_numa_masks_set(unsigned int cpu)
>  	int node = cpu_to_node(cpu);
>  	int i, j;
> 
> +	__sched_domains_numa_masks_set(node);
> +
>  	for (i = 0; i < sched_domains_numa_levels; i++) {
>  		for (j = 0; j < nr_node_ids; j++) {
> +			if (!node_online(j))
> +				continue;
> +
> +			/* Set ourselves in the remote node's masks */
>  			if (node_distance(j, node) <= sched_domains_numa_distance[i])
>  				cpumask_set_cpu(cpu, sched_domains_numa_masks[i][j]);
>  		}
> -- 
> 2.25.1
> 

-- 
Thanks and Regards
Srikar Dronamraju