[PATCH V3 1/2] sched/fair: Fix load_balance() affinity redo path

[Jeffrey Hugo]

If load_balance() fails to migrate any tasks because all tasks were
affined, load_balance() removes the source cpu from consideration and
attempts to redo and balance among the new subset of cpus.

There is a bug in this code path where the algorithm considers all active
cpus in the system (minus the source that was just masked out).  This is
not valid for two reasons: some active cpus may not be in the current
scheduling domain and one of the active cpus is dst_cpu. These cpus should
not be considered, as we cannot pull load from them.

Instead of failing out of load_balance(), we may end up redoing the search
with no valid cpus and incorrectly concluding the domain is balanced.
Additionally, if the group_imbalance flag was just set, it may also be
incorrectly unset, thus the flag will not be seen by other cpus in future
load_balance() runs as that algorithm intends.

Fix the check by removing cpus not in the current domain and the dst_cpu
from considertation, thus limiting the evaluation to valid remaining cpus
from which load might be migrated.

Co-authored-by: Austin Christ <austinwc@xxxxxxxxxxxxxx>
Co-authored-by: Dietmar Eggemann <dietmar.eggemann@xxxxxxx>
Signed-off-by: Jeffrey Hugo <jhugo@xxxxxxxxxxxxxx>
Tested-by: Tyler Baicar <tbaicar@xxxxxxxxxxxxxx>
---
 kernel/sched/fair.c | 19 ++++++++++++++++++-
 1 file changed, 18 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index d711093..a5d41b1 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -8220,7 +8220,24 @@ static int load_balance(int this_cpu, struct rq *this_rq,
 		/* All tasks on this runqueue were pinned by CPU affinity */
 		if (unlikely(env.flags & LBF_ALL_PINNED)) {
 			cpumask_clear_cpu(cpu_of(busiest), cpus);
-			if (!cpumask_empty(cpus)) {
+			/*
+			 * dst_cpu is not a valid busiest cpu in the following
+			 * check since load cannot be pulled from dst_cpu to be
+			 * put on dst_cpu.
+			 */
+			cpumask_clear_cpu(env.dst_cpu, cpus);
+			/*
+			 * Go back to "redo" iff the load-balance cpumask
+			 * contains other potential busiest cpus for the
+			 * current sched domain.
+			 */
+			if (cpumask_intersects(cpus, sched_domain_span(env.sd))) {
+				/*
+				 * Now that the check has passed, reenable
+				 * dst_cpu so that load can be calculated on
+				 * it in the redo path.
+				 */
+				cpumask_set_cpu(env.dst_cpu, cpus);
 				env.loop = 0;
 				env.loop_break = sched_nr_migrate_break;
 				goto redo;
-- 
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