[tip:sched/urgent] sched/fair: Fix load_balance() affinity redo path

From: tip-bot for Jeffrey Hugo
Date: Wed Jul 05 2017 - 18:53:02 EST

Commit-ID: 65a4433aebe36c8c6abeb69b99ef00274b971c6c
Gitweb: http://git.kernel.org/tip/65a4433aebe36c8c6abeb69b99ef00274b971c6c
Author: Jeffrey Hugo <jhugo@xxxxxxxxxxxxxx>
AuthorDate: Wed, 7 Jun 2017 13:18:57 -0600
Committer: Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Wed, 5 Jul 2017 16:28:48 +0200

sched/fair: Fix load_balance() affinity redo path

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>
Tested-by: Tyler Baicar <tbaicar@xxxxxxxxxxxxxx>
Signed-off-by: Jeffrey Hugo <jhugo@xxxxxxxxxxxxxx>
Acked-by: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
Cc: Austin Christ <austinwc@xxxxxxxxxxxxxx>
Cc: Dietmar Eggemann <dietmar.eggemann@xxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Timur Tabi <timur@xxxxxxxxxxxxxx>
Link: http://lkml.kernel.org/r/1496863138-11322-2-git-send-email-jhugo@xxxxxxxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
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 008c514..c95880e 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6646,10 +6646,10 @@ 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 */
@@ -8022,14 +8022,7 @@ 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);


@@ -8151,7 +8144,15 @@ more_balance:
/* 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;
@@ -8447,6 +8448,13 @@ 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,