Messages in this thread Patch in this message | | | From | Frederic Weisbecker <> | Subject | [PATCH 07/10] rcu/nocb: Limit number of softirq callbacks only on softirq | Date | Tue, 19 Oct 2021 02:08:13 +0200 |
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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> --- 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
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