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rcu: Defer printing stall-warning backtrace when holding rcu_node lock

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The rcu_dump_cpu_stacks() holds the leaf rcu_node structure's ->lock
when dumping the stakcks of any CPUs stalling the current grace period.
This lock is held to prevent confusion that would otherwise occur when
the stalled CPU reported its quiescent state (and then went on to do
unrelated things) just as the backtrace NMI was heading towards it.

This has worked well, but on larger systems has recently been observed
to cause severe lock contention resulting in CSD-lock stalls and other
general unhappiness.

This commit therefore does printk_deferred_enter() before acquiring
the lock and printk_deferred_exit() after releasing it, thus deferring
the overhead of actually outputting the stack trace out of that lock's
critical section.

Reported-by: Rik van Riel <riel@surriel.com>
Suggested-by: Rik van Riel <riel@surriel.com>
Signed-off-by: "Paul E. McKenney" <paulmck@kernel.org>
Signed-off-by: Neeraj Upadhyay <neeraj.upadhyay@kernel.org>
This commit is contained in:
Paul E. McKenney 2024-08-23 14:15:12 -07:00 committed by Neeraj Upadhyay
parent 51b739990c
commit 1ecd9d68eb
1 changed files with 2 additions and 0 deletions
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2
kernel/rcu/tree_stall.h
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@ -371,6 +371,7 @@ static void rcu_dump_cpu_stacks(void)
struct rcu_node *rnp;
rcu_for_each_leaf_node(rnp) {
printk_deferred_enter();
raw_spin_lock_irqsave_rcu_node(rnp, flags);
for_each_leaf_node_possible_cpu(rnp, cpu)
if (rnp->qsmask & leaf_node_cpu_bit(rnp, cpu)) {
@ -380,6 +381,7 @@ static void rcu_dump_cpu_stacks(void)
dump_cpu_task(cpu);
}
raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
printk_deferred_exit();
}
}