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Subject[PATCH rcu 11/18] rcu: Apply callbacks processing time limit only on softirq
Date
From: Frederic Weisbecker <frederic@kernel.org>

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>
---
kernel/rcu/tree.c | 25 ++++++++++++-------------
1 file changed, 12 insertions(+), 13 deletions(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 74e210ccfd382..1b31fad9f1a0a 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -2488,7 +2488,7 @@ static void rcu_do_batch(struct rcu_data *rdp)
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;
@@ -2528,6 +2528,17 @@ static void rcu_do_batch(struct rcu_data *rdp)
if (in_serving_softirq()) {
if (count >= bl && (need_resched() || !is_idle_task(current)))
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((count & 31) || local_clock() < tlimit))
+ continue;
+ /* Exceeded the time limit, so leave. */
+ break;
+ }
} else {
local_bh_enable();
lockdep_assert_irqs_enabled();
@@ -2535,18 +2546,6 @@ static void rcu_do_batch(struct rcu_data *rdp)
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))
- continue;
- /* Exceeded the time limit, so leave. */
- break;
- }
}

rcu_nocb_lock_irqsave(rdp, flags);
--
2.31.1.189.g2e36527f23
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