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 <austi...@codeaurora.org>
Co-authored-by: Dietmar Eggemann <dietmar.eggem...@arm.com>
Signed-off-by: Jeffrey Hugo <jh...@codeaurora.org>
Tested-by: Tyler Baicar <tbai...@codeaurora.org>
---
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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Qualcomm Technologies, Inc. is a member of the
Code Aurora Forum, a Linux Foundation Collaborative Project.
