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Date: Tue, 10 Aug 2021 17:17:27 +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>,
Michael Ellerman <mpe@...erman.id.au>,
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>,
Gautham R Shenoy <ego@...ux.vnet.ibm.com>,
Geetika Moolchandani <Geetika.Moolchandani1@....com>,
Laurent Dufour <ldufour@...ux.ibm.com>
Subject: Re: [PATCH v2 1/2] sched/topology: Skip updating masks for
non-online nodes
* Valentin Schneider <valentin.schneider@....com> [2021-08-09 13:52:38]:
> On 09/08/21 12:22, Srikar Dronamraju wrote:
> > * Valentin Schneider <valentin.schneider@....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@...ux.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@....com>
> Signed-off-by: Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
> Signed-off-by: Valentin Schneider <valentin.schneider@....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
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