Re: [PATCH RFC] sched: Allow migrating kthreads into online but inactive CPUs

[Peter Zijlstra]

Hi,

On Sat, Jun 17, 2017 at 08:10:08AM -0400, Tejun Heo wrote:
> 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@xxxxxxxxxx>
> Reported-by: "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx>
> Reported-by: Steven Rostedt <rostedt@xxxxxxxxxxx>

Hmm.. so the rules for running on !active && online are slightly
stricter than just being a kthread, how about the below, does that work
too?

 kernel/sched/core.c | 36 ++++++++++++++++++++++++++++++------
 1 file changed, 30 insertions(+), 6 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index d3d39a283beb..59b667c16826 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -894,6 +894,22 @@ void check_preempt_curr(struct rq *rq, struct task_struct *p, int flags)
 }
 
 #ifdef CONFIG_SMP
+
+/*
+ * Per-CPU kthreads are allowed to run on !actie && online CPUs, see
+ * __set_cpus_allowed_ptr() and select_fallback_rq().
+ */
+static inline bool is_per_cpu_kthread(struct task_struct *p)
+{
+	if (!(p->flags & PF_KTHREAD))
+		return false;
+
+	if (p->nr_cpus_allowed != 1)
+		return false;
+
+	return true;
+}
+
 /*
  * This is how migration works:
  *
@@ -951,8 +967,13 @@ 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 (is_per_cpu_kthread(p)) {
+		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))
@@ -1482,10 +1503,13 @@ static int select_fallback_rq(int cpu, struct task_struct *p)
 	for (;;) {
 		/* Any allowed, online CPU? */
 		for_each_cpu(dest_cpu, &p->cpus_allowed) {
-			if (!(p->flags & PF_KTHREAD) && !cpu_active(dest_cpu))
-				continue;
-			if (!cpu_online(dest_cpu))
-				continue;
+			if (is_per_cpu_kthread(p)) {
+				if (!cpu_online(dest_cpu))
+					continue;
+			} else {
+				if (!cpu_active(dest_cpu))
+					continue;
+			}
 			goto out;
 		}