From: Frederic Weisbecker <frederic@kernel.org>
To: "Paul E . McKenney" <paulmck@kernel.org>
Cc: LKML <linux-kernel@vger.kernel.org>,
Frederic Weisbecker <frederic@kernel.org>,
Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
Valentin Schneider <Valentin.Schneider@arm.com>,
Peter Zijlstra <peterz@infradead.org>,
Uladzislau Rezki <urezki@gmail.com>,
Thomas Gleixner <tglx@linutronix.de>,
Valentin Schneider <valentin.schneider@arm.com>,
Boqun Feng <boqun.feng@gmail.com>,
Neeraj Upadhyay <neeraju@codeaurora.org>,
Josh Triplett <josh@joshtriplett.org>,
Joel Fernandes <joel@joelfernandes.org>,
rcu@vger.kernel.org
Subject: [PATCH 08/11] rcu/nocb: Limit number of softirq callbacks only on softirq
Date: Mon, 11 Oct 2021 16:51:37 +0200 [thread overview]
Message-ID: <20211011145140.359412-9-frederic@kernel.org> (raw)
In-Reply-To: <20211011145140.359412-1-frederic@kernel.org>
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
next prev parent reply other threads:[~2021-10-11 14:52 UTC|newest]
Thread overview: 26+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-10-11 14:51 [PATCH 00/11] rcu: Make rcu_core() safe in PREEMPT_RT with NOCB + a few other fixes v2 Frederic Weisbecker
2021-10-11 14:51 ` [PATCH 01/11] rcu/nocb: Make local rcu_nocb_lock_irqsave() safe against concurrent deoffloading Frederic Weisbecker
2021-10-11 14:51 ` [PATCH 02/11] rcu/nocb: Prepare state machine for a new step Frederic Weisbecker
2021-10-11 14:51 ` [PATCH 03/11] rcu/nocb: Invoke rcu_core() at the start of deoffloading Frederic Weisbecker
2021-10-13 16:07 ` Boqun Feng
2021-10-14 11:07 ` Frederic Weisbecker
2021-10-14 11:42 ` Valentin Schneider
2021-10-14 13:57 ` Boqun Feng
2021-10-11 14:51 ` [PATCH 04/11] rcu/nocb: Make rcu_core() callbacks acceleration preempt-safe Frederic Weisbecker
2021-10-11 14:51 ` [PATCH 05/11] rcu/nocb: Make rcu_core() callbacks acceleration (de-)offloading safe Frederic Weisbecker
2021-10-11 14:51 ` [PATCH 06/11] rcu/nocb: Check a stable offloaded state to manipulate qlen_last_fqs_check Frederic Weisbecker
2021-10-11 14:51 ` [PATCH 07/11] rcu/nocb: Use appropriate rcu_nocb_lock_irqsave() Frederic Weisbecker
2021-10-11 14:51 ` Frederic Weisbecker [this message]
2021-10-11 14:51 ` [PATCH 09/11] rcu: Fix callbacks processing time limit retaining cond_resched() Frederic Weisbecker
2021-10-11 14:51 ` [PATCH 10/11] rcu: Apply callbacks processing time limit only on softirq Frederic Weisbecker
2021-10-11 14:51 ` [PATCH 11/11] rcu/nocb: Don't invoke local rcu core on callback overload from nocb kthread Frederic Weisbecker
2021-10-13 0:32 ` [PATCH 00/11] rcu: Make rcu_core() safe in PREEMPT_RT with NOCB + a few other fixes v2 Paul E. McKenney
2021-10-13 3:28 ` Paul E. McKenney
2021-10-13 10:01 ` Frederic Weisbecker
2021-10-13 11:43 ` Frederic Weisbecker
2021-10-13 16:27 ` Paul E. McKenney
2021-10-14 10:43 ` Frederic Weisbecker
-- strict thread matches above, loose matches on Subject: below --
2021-09-29 22:10 [PATCH 00/11] rcu: Make rcu_core() safe in PREEMPT_RT with NOCB + a few other fixes Frederic Weisbecker
2021-09-29 22:10 ` [PATCH 08/11] rcu/nocb: Limit number of softirq callbacks only on softirq Frederic Weisbecker
2021-10-01 17:51 ` Valentin Schneider
2021-10-04 13:42 ` Frederic Weisbecker
2021-10-05 20:55 ` Paul E. McKenney
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