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SubjectRe: [PATCH 1/2] workqueue: Move wq_update_unbound_numa() to the beginning of CPU_ONLINE

So, the issue of the initial worker not having its affinity set
correctly wasn't caused by the order of the operations. Reordering
just made set_cpus_allowed tried one more time late enough so that it
hides the race condition most of the time. The problem is that
CPU_ONLINE callbacks are called while the cpu being onlined is online
but not active and select_fallback_rq() only considers active cpus, so
if a kthread gets scheduled in the meantime and it doesn't have any
cpu which is active in its allowed mask, it's allowed mask gets reset
to cpu_possible_mask.

Would something like the following make sense?

------ 8< ------
Subject: [PATCH] sched: allow kthreads to fallback to online && !active cpus

During CPU hotplug, CPU_ONLINE callbacks are run while the CPU is
online but not active. A CPU_ONLINE callback may create or bind a
kthread so that its cpus_allowed mask only allows the CPU which is
being brought online. The kthread may start executing before the CPU
is made active and can end up in select_fallback_rq().

In such cases, the expected behavior is selecting the CPU which is
coming online; however, because select_fallback_rq() only chooses from
active CPUs, it determines that the task doesn't have any viable CPU
in its allowed mask and ends up overriding it to cpu_possible_mask.

CPU_ONLINE callbacks should be able to put kthreads on the CPU which
is coming online. Update select_fallback_rq() so that it follows
cpu_online() rather than cpu_active() for kthreads.

Signed-off-by: Tejun Heo <>
Reported-by: Gautham R Shenoy <>
kernel/sched/core.c | 4 +++-
1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 017d539..a12e3db 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -1536,7 +1536,9 @@ static int select_fallback_rq(int cpu, struct task_struct *p)
for (;;) {
/* Any allowed, online CPU? */
for_each_cpu(dest_cpu, tsk_cpus_allowed(p)) {
- if (!cpu_active(dest_cpu))
+ if (!(p->flags & PF_KTHREAD) && !cpu_active(dest_cpu))
+ continue;
+ if (!cpu_online(dest_cpu))
goto out;
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