[PATCH 5.15 104/135] rcu: Fix callbacks processing time limit retaining cond_resched()

Greg Kroah-Hartman
2022-05-10T14:18:54Z

From: Frederic Weisbecker <frederic@kernel.org>

commit 3e61e95e2d095e308616cba4ffb640f95a480e01 upstream.

The callbacks processing time limit makes sure we are not exceeding a
given amount of time executing the queue.

However its "continue" clause bypasses the cond_resched() call on
rcuc and NOCB kthreads, delaying it until we reach the limit, which can
be very long...

Make sure the scheduler has a higher priority than the time limit.

Reviewed-by: Valentin Schneider <valentin.schneider@arm.com>
Tested-by: Valentin Schneider <valentin.schneider@arm.com>
Tested-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Cc: Valentin Schneider <valentin.schneider@arm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Josh Triplett <josh@joshtriplett.org>
Cc: Joel Fernandes <joel@joelfernandes.org>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Neeraj Upadhyay <neeraju@codeaurora.org>
Cc: Uladzislau Rezki <urezki@gmail.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
[UR: backport to 5.15-stable + commit update]
Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
kernel/rcu/tree.c | 27 ++++++++++++++++-----------
1 file changed, 16 insertions(+), 11 deletions(-)

--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -2513,10 +2513,22 @@ static void rcu_do_batch(struct rcu_data
/*
* Stop only if limit reached and CPU has something to do.
*/
- if (count >= bl && !offloaded &&
- (need_resched() ||
- (!is_idle_task(current) && !rcu_is_callbacks_kthread())))
- break;
+ if (in_serving_softirq()) {
+ if (count >= bl && (need_resched() ||
+ (!is_idle_task(current) && !rcu_is_callbacks_kthread())))
+ break;
+ } else {
+ local_bh_enable();
+ lockdep_assert_irqs_enabled();
+ cond_resched_tasks_rcu_qs();
+ lockdep_assert_irqs_enabled();
+ local_bh_disable();
+ }
+
+ /*
+ * Make sure we don't spend too much time here and deprive other
+ * softirq vectors of CPU cycles.
+ */
if (unlikely(tlimit)) {
/* only call local_clock() every 32 callbacks */
if (likely((count & 31) || local_clock() < tlimit))
@@ -2524,13 +2536,6 @@ static void rcu_do_batch(struct rcu_data
/* Exceeded the time limit, so leave. */
break;
}
- if (!in_serving_softirq()) {
- local_bh_enable();
- lockdep_assert_irqs_enabled();
- cond_resched_tasks_rcu_qs();
- lockdep_assert_irqs_enabled();
- local_bh_disable();
- }
}

local_irq_save(flags);