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From: "Joel Fernandes (Google)" <joel@joelfernandes.org>
To: linux-kernel@vger.kernel.org
Cc: "Joel Fernandes (Google)" <joel@joelfernandes.org>,
	Josh Triplett <josh@joshtriplett.org>,
	Lai Jiangshan <jiangshanlai@gmail.com>,
	Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
	"Paul E. McKenney" <paulmck@linux.ibm.com>,
	rcu@vger.kernel.org, Steven Rostedt <rostedt@goodmis.org>
Subject: [RFC v2] rcu/tree: Try to invoke_rcu_core() if in_irq() during unlock
Date: Sun, 18 Aug 2019 17:49:48 -0400
Message-ID: <20190818214948.GA134430@google.com> (raw)

When we're in hard interrupt context in rcu_read_unlock_special(), we
can still benefit from invoke_rcu_core() doing wake ups of rcuc
threads when the !use_softirq parameter is passed.  This is safe
to do so because:

1. We avoid the scheduler deadlock issues thanks to the deferred_qs bit
introduced in commit 23634ebc1d94 ("rcu: Check for wakeup-safe
conditions in rcu_read_unlock_special()") by checking for the same in
this patch.

2. in_irq() implies in_interrupt() which implies raising softirq will
not do any wake ups.

The rcuc thread which is awakened will run when the interrupt returns.

We also honor 25102de ("rcu: Only do rcu_read_unlock_special() wakeups
if expedited") thus doing the rcuc awakening only when none of the
following are true:
  1. Critical section is blocking an expedited GP.
  2. A nohz_full CPU.
If neither of these cases are true (exp == false), then the "else" block
will run to do the irq_work stuff.

This commit is based on a partial revert of d143b3d1cd89 ("rcu: Simplify
rcu_read_unlock_special() deferred wakeups") with an additional in_irq()
check added.

Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>

---
v1->v2: Some minor character encoding issues in changelog corrected.

Note that I am still testing this patch, but I sent an early RFC for your
feedback. Thanks!

 kernel/rcu/tree_plugin.h | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 2defc7fe74c3..f4b3055026dc 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -621,6 +621,11 @@ static void rcu_read_unlock_special(struct task_struct *t)
 			// Using softirq, safe to awaken, and we get
 			// no help from enabling irqs, unlike bh/preempt.
 			raise_softirq_irqoff(RCU_SOFTIRQ);
+		} else if (exp && in_irq() && !use_softirq &&
+			   !t->rcu_read_unlock_special.b.deferred_qs) {
+			// Safe to awaken rcuc kthread which will be
+                       // scheduled in from the interrupt return path.
+			invoke_rcu_core();
 		} else {
 			// Enabling BH or preempt does reschedule, so...
 			// Also if no expediting or NO_HZ_FULL, slow is OK.
-- 
2.23.0.rc1.153.gdeed80330f-goog


             reply index

Thread overview: 23+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-08-18 21:49 Joel Fernandes (Google) [this message]
2019-08-18 22:12 ` Paul E. McKenney
2019-08-18 22:32   ` Joel Fernandes
2019-08-18 22:35     ` Joel Fernandes
2019-08-18 23:31       ` Paul E. McKenney
2019-08-18 23:38         ` Joel Fernandes
2019-08-19  1:21           ` Paul E. McKenney
2019-08-19  1:41             ` Joel Fernandes
2019-08-19  1:46               ` Joel Fernandes
2019-08-19  2:29                 ` Paul E. McKenney
2019-08-19 12:57                   ` Paul E. McKenney
2019-08-19 14:33                     ` Paul E. McKenney
2019-08-19 15:41                       ` Paul E. McKenney
2019-08-19 16:25                         ` Joel Fernandes
2019-08-21 14:38                         ` Joel Fernandes
2019-08-21 14:56                           ` Joel Fernandes
2019-08-21 15:26                             ` Joel Fernandes
2019-08-21 15:47                               ` Paul E. McKenney
2019-08-21 15:39                             ` Paul E. McKenney
2019-08-21 15:46                               ` Joel Fernandes
2019-08-21 15:26                           ` Paul E. McKenney
2019-08-20  0:14 ` Scott Wood
2019-08-20  1:40   ` Joel Fernandes

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