Offlining offloaded CPUs don't migrate their callbacks just like non-offloaded CPUs do. It's up to their CB/GP kthread to handle what remains.
Therefore we can't afford to de-offload an offline CPU that still has pending work to do, or the callbacks would be ignored. NOTE: The long term solution will be to wait for all pending callbacks to be processed before completing a CPU down operation. Suggested-by: Paul E. McKenney <paul...@kernel.org> Signed-off-by: Frederic Weisbecker <frede...@kernel.org> Cc: Paul E. McKenney <paul...@kernel.org> Cc: Josh Triplett <j...@joshtriplett.org> Cc: Steven Rostedt <rost...@goodmis.org> Cc: Mathieu Desnoyers <mathieu.desnoy...@efficios.com> Cc: Lai Jiangshan <jiangshan...@gmail.com> Cc: Joel Fernandes <j...@joelfernandes.org> Cc: Neeraj Upadhyay <neer...@codeaurora.org> Cc: Thomas Gleixner <t...@linutronix.de> Cc: Boqun Feng <boqun.f...@gmail.com> --- kernel/rcu/tree_plugin.h | 9 +++++++++ 1 file changed, 9 insertions(+) diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h index 08eb035da9e7..5075bf219b23 100644 --- a/kernel/rcu/tree_plugin.h +++ b/kernel/rcu/tree_plugin.h @@ -2227,6 +2227,15 @@ static int __rcu_nocb_rdp_deoffload(struct rcu_data *rdp) printk("De-offloading %d\n", rdp->cpu); rcu_nocb_lock_irqsave(rdp, flags); + /* + * If there are still pending work offloaded, the offline + * CPU won't help much handling them. + */ + if (cpu_is_offline(rdp->cpu) && !rcu_segcblist_empty(&rdp->cblist)) { + rcu_nocb_unlock_irqrestore(rdp, flags); + return -EBUSY; + } + rcu_segcblist_offload(cblist, false); if (rdp->nocb_cb_sleep) { -- 2.25.1