Commit 955dbdf4 authored Jun 17, 2017 by Tejun Heo Committed by Paul E. McKenney Jul 28, 2017

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@kernel.org>
Reported-by: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Reported-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

parent 4e327470

kernel/sched/core.c

+7 −2

Original line number	Diff line number	Diff line
		@@ -951,8 +951,13 @@ struct migration_arg {
		static struct rq __migrate_task(struct rq rq, struct rq_flags *rf,
		struct task_struct *p, int dest_cpu)
		{
		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))

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