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rcu/nocb: Don't deoffload an offline CPU with pending work

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Offloaded CPUs do not migrate their callbacks, instead relying on
their rcuo kthread to invoke them.  But if the CPU is offline, it
will be running neither its RCU_SOFTIRQ handler nor its rcuc kthread.
This means that de-offloading an offline CPU that still has pending
callbacks will strand those callbacks.  This commit therefore refuses
to toggle offline CPUs having pending callbacks.

Cc: Josh Triplett <josh@joshtriplett.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Lai Jiangshan <jiangshanlai@gmail.com>
Cc: Joel Fernandes <joel@joelfernandes.org>
Cc: Neeraj Upadhyay <neeraju@codeaurora.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Suggested-by: Paul E. McKenney <paulmck@kernel.org>
Tested-by: Boqun Feng <boqun.feng@gmail.com>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
This commit is contained in:
Frederic Weisbecker 2020-11-13 13:13:20 +01:00 committed by Paul E. McKenney
parent d97b078182
commit ef005345e6
1 changed files with 9 additions and 0 deletions
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9
kernel/rcu/tree_plugin.h
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@ -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) {