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rcu/nocb: Limit number of softirq callbacks only on softirq

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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>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
This commit is contained in:
Frederic Weisbecker 2021-10-19 02:08:13 +02:00 committed by Paul E. McKenney
parent 7b65dfa32d
commit 78ad37a2c5
1 changed files with 2 additions and 3 deletions
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5
kernel/rcu/tree.c
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@ -2525,9 +2525,8 @@ 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 */