This is a note to let you know that I've just added the patch titled
sched/fair: Fix load_balance() affinity redo path
to the 4.12-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
sched-fair-fix-load_balance-affinity-redo-path.patch
and it can be found in the queue-4.12 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
>From 65a4433aebe36c8c6abeb69b99ef00274b971c6c Mon Sep 17 00:00:00 2001
From: Jeffrey Hugo <jhugo@xxxxxxxxxxxxxx>
Date: Wed, 7 Jun 2017 13:18:57 -0600
Subject: sched/fair: Fix load_balance() affinity redo path
From: Jeffrey Hugo <jhugo@xxxxxxxxxxxxxx>
commit 65a4433aebe36c8c6abeb69b99ef00274b971c6c upstream.
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>
Signed-off-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
kernel/sched/fair.c | 32 ++++++++++++++++++++------------
1 file changed, 20 insertions(+), 12 deletions(-)
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6619,10 +6619,10 @@ int can_migrate_task(struct task_struct
* 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 */
@@ -7973,14 +7973,7 @@ static int load_balance(int this_cpu, st
.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]);
@@ -8102,7 +8095,15 @@ 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;
@@ -8398,6 +8399,13 @@ static int active_load_balance_cpu_stop(
.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);
Patches currently in stable-queue which might be from jhugo@xxxxxxxxxxxxxx are
queue-4.12/sched-fair-fix-load_balance-affinity-redo-path.patch