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rcu: Finer-grained grace-period-end checks in rcu_dump_cpu_stacks()

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This commit pushes the grace-period-end checks further down into
rcu_dump_cpu_stacks(), and also uses lockless checks coupled with
finer-grained locking.

The result is that the current leaf rcu_node structure's ->lock is
acquired only if a stack backtrace might be needed from the current CPU,
and is held across only that CPU's backtrace.  As a result, if there are
no stalled CPUs associated with a given rcu_node structure, then its
->lock will not be acquired at all.  On large systems, it is usually
(though not always) the case that a small number of CPUs are stalling
the current grace period, which means that the ->lock need be acquired
only for a small fraction of the rcu_node structures.

[ paulmck: Apply Dan Carpenter feedback. ]

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
This commit is contained in:
Paul E. McKenney 2024-10-28 17:22:51 -07:00 committed by Frederic Weisbecker
parent cbe644aa6f
commit 9650edd9bf
1 changed files with 11 additions and 7 deletions
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8
kernel/rcu/tree_stall.h
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@ -342,13 +342,16 @@ static void rcu_dump_cpu_stacks(unsigned long gp_seq)
struct rcu_node *rnp;
rcu_for_each_leaf_node(rnp) {
printk_deferred_enter();
for_each_leaf_node_possible_cpu(rnp, cpu) {
if (gp_seq != data_race(rcu_state.gp_seq)) {
printk_deferred_exit();
pr_err("INFO: Stall ended during stack backtracing.\n");
return;
}
printk_deferred_enter();
if (!(data_race(rnp->qsmask) & leaf_node_cpu_bit(rnp, cpu)))
continue;
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)) {
if (cpu_is_offline(cpu))
pr_err("Offline CPU %d blocking current GP.\n", cpu);
@ -356,6 +359,7 @@ static void rcu_dump_cpu_stacks(unsigned long gp_seq)
dump_cpu_task(cpu);
}
raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
}
printk_deferred_exit();
}
}