sched/topology: Fix overlapping sched_group_mask

The point of sched_group_mask is to select those CPUs from
sched_group_cpus that can actually arrive at this balance domain.

The current code gets it wrong, as can be readily demonstrated with a
topology like:

  node   0   1   2   3
    0:  10  20  30  20
    1:  20  10  20  30
    2:  30  20  10  20
    3:  20  30  20  10

Where (for example) domain 1 on CPU1 ends up with a mask that includes
CPU0:

  [] CPU1 attaching sched-domain:
  []  domain 0: span 0-2 level NUMA
  []   groups: 1 (mask: 1), 2, 0
  []   domain 1: span 0-3 level NUMA
  []    groups: 0-2 (mask: 0-2) (cpu_capacity: 3072), 0,2-3 (cpu_capacity: 3072)

This causes sched_balance_cpu() to compute the wrong CPU and
consequently should_we_balance() will terminate early resulting in
missed load-balance opportunities.

The fixed topology looks like:

  [] CPU1 attaching sched-domain:
  []  domain 0: span 0-2 level NUMA
  []   groups: 1 (mask: 1), 2, 0
  []   domain 1: span 0-3 level NUMA
  []    groups: 0-2 (mask: 1) (cpu_capacity: 3072), 0,2-3 (cpu_capacity: 3072)

(note: this relies on OVERLAP domains to always have children, this is
 true because the regular topology domains are still here -- this is
 before degenerate trimming)

Debugged-by: Lauro Ramos Venancio <lvenanci@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-kernel@vger.kernel.org
Cc: stable@vger.kernel.org
Fixes: e3589f6c81e4 ("sched: Allow for overlapping sched_domain spans")
Signed-off-by: Ingo Molnar <mingo@kernel.org>

diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
index 21efacf547d4865e8b999c943c97e72d1687e054..09563c1d1d5b4b7cb39f2e410bf0660d8418194a 100644 (file)
--- a/kernel/sched/topology.c
+++ b/kernel/sched/topology.c
@@ -495,6 +495,9 @@ enum s_alloc {
 /*
  * Build an iteration mask that can exclude certain CPUs from the upwards
  * domain traversal.
+ *
+ * Only CPUs that can arrive at this group should be considered to continue
+ * balancing.
  */
 static void build_group_mask(struct sched_domain *sd, struct sched_group *sg)
 {
@@ -505,11 +508,24 @@ static void build_group_mask(struct sched_domain *sd, struct sched_group *sg)
 
        for_each_cpu(i, sg_span) {
                sibling = *per_cpu_ptr(sdd->sd, i);
-               if (!cpumask_test_cpu(i, sched_domain_span(sibling)))
+
+               /*
+                * Can happen in the asymmetric case, where these siblings are
+                * unused. The mask will not be empty because those CPUs that
+                * do have the top domain _should_ span the domain.
+                */
+               if (!sibling->child)
+                       continue;
+
+               /* If we would not end up here, we can't continue from here */
+               if (!cpumask_equal(sg_span, sched_domain_span(sibling->child)))
                        continue;
 
                cpumask_set_cpu(i, sched_group_mask(sg));
        }
+
+       /* We must not have empty masks here */
+       WARN_ON_ONCE(cpumask_empty(sched_group_mask(sg)));
 }
 
 /*