[PATCH 5.10 65/70] rcu: Apply callbacks processing time limit only on softirq

Greg Kroah-Hartman
2022-05-10T13:56:08Z

From: Frederic Weisbecker <frederic@kernel.org>

commit a554ba288845fd3f6f12311fd76a51694233458a upstream.

Time limit only makes sense when callbacks are serviced in softirq mode
because:

_ In case we need to get back to the scheduler,
cond_resched_tasks_rcu_qs() is called after each callback.

_ In case some other softirq vector needs the CPU, the call to
local_bh_enable() before cond_resched_tasks_rcu_qs() takes care about
them via a call to do_softirq().

Therefore, make sure the time limit only applies to softirq mode.

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 5.10-stable]
Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
kernel/rcu/tree.c | 26 +++++++++++++-------------
1 file changed, 13 insertions(+), 13 deletions(-)

--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -2456,7 +2456,7 @@ static void rcu_do_batch(struct rcu_data
div = READ_ONCE(rcu_divisor);
div = div < 0 ? 7 : div > sizeof(long) * 8 - 2 ? sizeof(long) * 8 - 2 : div;
bl = max(rdp->blimit, pending >> div);
- if (unlikely(bl > 100)) {
+ if (in_serving_softirq() && unlikely(bl > 100)) {
long rrn = READ_ONCE(rcu_resched_ns);

rrn = rrn < NSEC_PER_MSEC ? NSEC_PER_MSEC : rrn > NSEC_PER_SEC ? NSEC_PER_SEC : rrn;
@@ -2494,6 +2494,18 @@ static void rcu_do_batch(struct rcu_data
if (-rcl.len >= bl && (need_resched() ||
(!is_idle_task(current) && !rcu_is_callbacks_kthread())))
break;
+
+ /*
+ * 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((-rcl.len & 31) || local_clock() < tlimit))
+ continue;
+ /* Exceeded the time limit, so leave. */
+ break;
+ }
} else {
local_bh_enable();
lockdep_assert_irqs_enabled();
@@ -2501,18 +2513,6 @@ static void rcu_do_batch(struct rcu_data
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((-rcl.len & 31) || local_clock() < tlimit))
- continue;
- /* Exceeded the time limit, so leave. */
- break;
- }
}

local_irq_save(flags);