Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]

Groups > linux.kernel > #1640618

[RFC 1/2] sched/fair: Fix load_balance() affinity redo path

Show all headers | View raw

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.

Signed-off-by: Austin Christ <austinwc@codeaurora.org>
Signed-off-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
Signed-off-by: Jeffrey Hugo <jhugo@codeaurora.org>
Tested-by: Tyler Baicar <tbaicar@codeaurora.org>
---
 kernel/sched/fair.c | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index d711093..8f783ba 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -8219,8 +8219,19 @@ 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)) {
+			struct cpumask tmp;
+
+			/* Cpumask of all initially possible busiest cpus. */
+			cpumask_copy(&tmp, sched_domain_span(env.sd));
+			cpumask_clear_cpu(env.dst_cpu, &tmp);
+
 			cpumask_clear_cpu(cpu_of(busiest), cpus);
-			if (!cpumask_empty(cpus)) {
+			/*
+			 * Go back to "redo" iff the load-balance cpumask
+			 * contains other potential busiest cpus for the
+			 * current sched domain.
+			 */
+			if (cpumask_intersects(cpus, &tmp)) {
 				env.loop = 0;
 				env.loop_break = sched_nr_migrate_break;
 				goto redo;
-- 
Qualcomm Datacenter Technologies as an affiliate of Qualcomm Technologies, Inc.
Qualcomm Technologies, Inc. is a member of the
Code Aurora Forum, a Linux Foundation Collaborative Project.

Back to linux.kernel | Previous | Next — Next in thread | Find similar | Unroll thread

Thread

[RFC 1/2] sched/fair: Fix load_balance() affinity redo path Jeffrey Hugo <jhugo@codeaurora.org> - 2017-05-12 19:10 +0200
  Re: [RFC 1/2] sched/fair: Fix load_balance() affinity redo path Jeffrey Hugo <jhugo@codeaurora.org> - 2017-05-12 19:30 +0200
    Re: [RFC 1/2] sched/fair: Fix load_balance() affinity redo path Peter Zijlstra <peterz@infradead.org> - 2017-05-12 22:50 +0200
      Re: [RFC 1/2] sched/fair: Fix load_balance() affinity redo path Jeffrey Hugo <jhugo@codeaurora.org> - 2017-05-12 23:00 +0200
  Re: [RFC 1/2] sched/fair: Fix load_balance() affinity redo path Peter Zijlstra <peterz@infradead.org> - 2017-05-12 19:30 +0200
  Re: [RFC 1/2] sched/fair: Fix load_balance() affinity redo path Peter Zijlstra <peterz@infradead.org> - 2017-05-12 22:50 +0200
    Re: [RFC 1/2] sched/fair: Fix load_balance() affinity redo path Jeffrey Hugo <jhugo@codeaurora.org> - 2017-05-12 23:00 +0200
      Re: [RFC 1/2] sched/fair: Fix load_balance() affinity redo path Dietmar Eggemann <dietmar.eggemann@arm.com> - 2017-05-15 17:00 +0200
        Re: [RFC 1/2] sched/fair: Fix load_balance() affinity redo path Jeffrey Hugo <jhugo@codeaurora.org> - 2017-05-18 16:40 +0200

csiph-web