[PATCH V5 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 | 32 ++++++++++++++++++++------------
 1 file changed, 20 insertions(+), 12 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index d711093..2ba4407 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6737,10 +6737,10 @@ int can_migrate_task(struct task_struct *p, struct lb_env *env)
 		 * our sched_group. We may want to revisit it if we couldn't
 		 * meet load balance goals by pulling other tasks on src_cpu.
 		 *
-		 * Also avoid computing new_dst_cpu if we have already computed
-		 * one in current iteration.
+		 * Avoid computing new_dst_cpu for NEWLY_IDLE or if we have
+		 * already computed one in current iteration.
 		 */
-		if (!env->dst_grpmask || (env->flags & LBF_DST_PINNED))
+		if (env->idle == CPU_NEWLY_IDLE || (env->flags & LBF_DST_PINNED))
 			return 0;
 
 		/* Prevent to re-select dst_cpu via env's cpus */
@@ -8091,14 +8091,7 @@ static int load_balance(int this_cpu, struct rq *this_rq,
 		.tasks		= LIST_HEAD_INIT(env.tasks),
 	};
 
-	/*
-	 * For NEWLY_IDLE load_balancing, we don't need to consider
-	 * other cpus in our group
-	 */
-	if (idle == CPU_NEWLY_IDLE)
-		env.dst_grpmask = NULL;
-
-	cpumask_copy(cpus, cpu_active_mask);
+	cpumask_and(cpus, sched_domain_span(sd), cpu_active_mask);
 
 	schedstat_inc(sd->lb_count[idle]);
 
@@ -8220,7 +8213,15 @@ 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)) {
+			/*
+			 * Attempting to continue load balancing at the current
+			 * sched_domain level only makes sense if there are
+			 * active cpus remaining as possible busiest cpus to
+			 * pull load from which are not contained within the
+			 * destination group that is receiving any migrated
+			 * load.
+			 */
+			if (!cpumask_subset(cpus, env.dst_grpmask)) {
 				env.loop = 0;
 				env.loop_break = sched_nr_migrate_break;
 				goto redo;
@@ -8516,6 +8517,13 @@ static int active_load_balance_cpu_stop(void *data)
 			.src_cpu	= busiest_rq->cpu,
 			.src_rq		= busiest_rq,
 			.idle		= CPU_IDLE,
+			/*
+			 * can_migrate_task() doesn't need to compute new_dst_cpu
+			 * for active balancing. Since we have CPU_IDLE, but no
+			 * @dst_grpmask we need to make that test go away with lying
+			 * about DST_PINNED.
+			 */
+			.flags		= LBF_DST_PINNED,
 		};
 
 		schedstat_inc(sd->alb_count);
-- 
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