[PATCH 08/11] rcu/nocb: Limit number of softirq callbacks only on softirq

Frederic Weisbecker
2021-10-11T14:52:30Z

The current condition to limit the number of callbacks executed in a
row checks the offloaded state of the rdp. Not only is it volatile
but it is also misleading: the rcu_core() may well be executing
callbacks concurrently with NOCB kthreads, and the offloaded state
would then be verified on both cases. As a result the limit would
spuriously not apply anymore on softirq while in the middle of
(de-)offloading process.

Fix and clarify the condition with those constraints in mind:

_ If callbacks are processed either by rcuc or NOCB kthread, the call
to cond_resched_tasks_rcu_qs() is enough to take care of the overload.

_ If instead callbacks are processed by softirqs:
* If need_resched(), exit the callbacks processing
* Otherwise if CPU is idle we can continue
* Otherwise exit because a softirq shouldn't interrupt a task for too
long nor deprive other pending softirq vectors of the CPU.

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>
---
kernel/rcu/tree.c | 5 ++---
1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index eaa9c7ce91bb..716dead1509d 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -2535,9 +2535,8 @@ static void rcu_do_batch(struct rcu_data *rdp)
/*
* 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())))
+ if (count >= bl && in_serving_softirq() &&
+ (need_resched() || !is_idle_task(current)))
break;
if (unlikely(tlimit)) {
/* only call local_clock() every 32 callbacks */
--
2.25.1