[PATCH 4.4 48/57] sched/topology: Fix overlapping sched_group_mask

Greg Kroah-Hartman
2017-07-19T11:14:49Z

From: Peter Zijlstra <peterz@infradead.org>

commit 73bb059f9b8a00c5e1bf2f7ca83138c05d05e600 upstream.

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
Fixes: e3589f6c81e4 ("sched: Allow for overlapping sched_domain spans")
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
kernel/sched/core.c | 18 +++++++++++++++++-
1 file changed, 17 insertions(+), 1 deletion(-)

--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -6123,6 +6123,9 @@ 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.
+ *
* Asymmetric node setups can result in situations where the domain tree is of
* unequal depth, make sure to skip domains that already cover the entire
* range.
@@ -6141,11 +6144,24 @@ static void build_group_mask(struct sche

for_each_cpu(i, 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)));
}

/*