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 <[email protected]>
> Reported-by: "Paul E. McKenney" <[email protected]>
> Reported-by: Steven Rostedt <[email protected]>
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;
}
