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@codeaurora.org>
Co-authored-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
Tested-by: Tyler Baicar <tbaicar@codeaurora.org>
Signed-off-by: Jeffrey Hugo <jhugo@codeaurora.org>
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Austin Christ <austinwc@codeaurora.org>
Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Timur Tabi <timur@codeaurora.org>
Link: http://lkml.kernel.org/r/1496863138-11322-2-git-send-email-jhugo@codeaurora.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 008c514..c95880e 100644 (file)
--- 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);
 
        schedstat_inc(sd->lb_count[idle]);
 
@@ -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,
                };
 
                schedstat_inc(sd->alb_count);