From: Jeffrey Hugo <jhugo@codeaurora.org>
To: Ingo Molnar <mingo@redhat.com>,
Peter Zijlstra <peterz@infradead.org>,
linux-kernel@vger.kernel.org
Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>,
Austin Christ <austinwc@codeaurora.org>,
Tyler Baicar <tbaicar@codeaurora.org>,
Jeffrey Hugo <jhugo@codeaurora.org>
Subject: [RFC 1/2] sched/fair: Fix load_balance() affinity redo path
Date: Fri, 12 May 2017 11:01:37 -0600 [thread overview]
Message-ID: <1494608498-4538-2-git-send-email-jhugo@codeaurora.org> (raw)
In-Reply-To: <1494608498-4538-1-git-send-email-jhugo@codeaurora.org>
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.
next prev parent reply other threads:[~2017-05-12 17:02 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-05-12 17:01 [RFC 0/2] load_balance() fixes for affinity Jeffrey Hugo
2017-05-12 17:01 ` Jeffrey Hugo [this message]
2017-05-12 17:23 ` [RFC 1/2] sched/fair: Fix load_balance() affinity redo path Peter Zijlstra
2017-05-12 17:29 ` Jeffrey Hugo
2017-05-12 20:44 ` Peter Zijlstra
2017-05-12 20:54 ` Jeffrey Hugo
2017-05-12 20:47 ` Peter Zijlstra
2017-05-12 20:57 ` Jeffrey Hugo
2017-05-15 14:56 ` Dietmar Eggemann
2017-05-18 14:31 ` Jeffrey Hugo
2017-05-12 17:01 ` [RFC 2/2] sched/fair: Remove group imbalance from calculate_imbalance() Jeffrey Hugo
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