linux-kernel - [PATCH RFC] sched: Allow migrating kthreads into online but inactive CPUs

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Message-ID: <20170617121008.GB20758@htj.duckdns.org>
Date:   Sat, 17 Jun 2017 08:10:08 -0400
From:   Tejun Heo <tj@...nel.org>
To:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        linux-kernel@...r.kernel.org,
        Lai Jiangshan <jiangshanlai@...il.com>, kernel-team@...com
Subject: [PATCH RFC] sched: Allow migrating kthreads into online but inactive
 CPUs

Per-cpu workqueues have been tripping CPU affinity sanity checks while
a CPU is being offlined.  A per-cpu kworker ends up running on a CPU
which isn't its target CPU while the CPU is online but inactive.

While the scheduler allows kthreads to wake up on an online but
inactive CPU, it doesn't allow a running kthread to be migrated to
such a CPU, which leads to an odd situation where setting affinity on
a sleeping and running kthread leads to different results.

Each mem-reclaim workqueue has one rescuer which guarantees forward
progress and the rescuer needs to bind itself to the CPU which needs
help in making forward progress; however, due to the above issue,
while set_cpus_allowed_ptr() succeeds, the rescuer doesn't end up on
the correct CPU if the CPU is in the process of going offline,
tripping the sanity check and executing the work item on the wrong
CPU.

This patch updates __migrate_task() so that kthreads can be migrated
into an inactive but online CPU.

Signed-off-by: Tejun Heo <tj@...nel.org>
Reported-by: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Reported-by: Steven Rostedt <rostedt@...dmis.org>
---
 kernel/sched/core.c |    9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 803c3bc274c4..1500217ce4b4 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -980,8 +980,13 @@ struct migration_arg {
 static struct rq *__migrate_task(struct rq *rq, struct rq_flags *rf,
 				 struct task_struct *p, int dest_cpu)
 {
-	if (unlikely(!cpu_active(dest_cpu)))
-		return rq;
+	if (p->flags & PF_KTHREAD) {
+		if (unlikely(!cpu_online(dest_cpu)))
+			return rq;
+	} else {
+		if (unlikely(!cpu_active(dest_cpu)))
+			return rq;
+	}

 	/* Affinity changed (again). */
 	if (!cpumask_test_cpu(dest_cpu, &p->cpus_allowed))