From 78ad37a2c50dfdb9a60e42bb9ee1da86d1fe770c Mon Sep 17 00:00:00 2001 From: Frederic Weisbecker Date: Tue, 19 Oct 2021 02:08:13 +0200 Subject: [PATCH] rcu/nocb: Limit number of softirq callbacks only on softirq 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 Tested-by: Sebastian Andrzej Siewior Signed-off-by: Frederic Weisbecker Cc: Valentin Schneider Cc: Peter Zijlstra Cc: Sebastian Andrzej Siewior Cc: Josh Triplett Cc: Joel Fernandes Cc: Boqun Feng Cc: Neeraj Upadhyay Cc: Uladzislau Rezki Cc: Thomas Gleixner Signed-off-by: Paul E. McKenney --- 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 20587d035d03..4f4d9ea6a9cb 100644 --- a/kernel/rcu/tree.c +++ b/kernel/rcu/tree.c @@ -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 */