linux-kernel - [PATCH rcu 5/9] rcu: handle quiescent states for PREEMPT_RCU=n, PREEMPT

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Message-Id: <20250116201327.3782963-5-paulmck@kernel.org>
Date: Thu, 16 Jan 2025 12:13:23 -0800
From: "Paul E. McKenney" <paulmck@...nel.org>
To: rcu@...r.kernel.org
Cc: linux-kernel@...r.kernel.org,
	kernel-team@...a.com,
	rostedt@...dmis.org,
	Ankur Arora <ankur.a.arora@...cle.com>,
	"Paul E . McKenney" <paulmck@...nel.org>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	Frederic Weisbecker <frederic@...nel.org>
Subject: [PATCH rcu 5/9] rcu: handle quiescent states for PREEMPT_RCU=n, PREEMPT_COUNT=y

From: Ankur Arora <ankur.a.arora@...cle.com>

With PREEMPT_RCU=n, cond_resched() provides urgently needed quiescent
states for read-side critical sections via rcu_all_qs().
One reason why this was needed: lacking preempt-count, the tick
handler has no way of knowing whether it is executing in a
read-side critical section or not.

With (PREEMPT_LAZY=y, PREEMPT_DYNAMIC=n), we get (PREEMPT_COUNT=y,
PREEMPT_RCU=n). In this configuration cond_resched() is a stub and
does not provide quiescent states via rcu_all_qs().
(PREEMPT_RCU=y provides this information via rcu_read_unlock() and
its nesting counter.)

So, use the availability of preempt_count() to report quiescent states
in rcu_flavor_sched_clock_irq().

Suggested-by: Paul E. McKenney <paulmck@...nel.org>
Reviewed-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Signed-off-by: Ankur Arora <ankur.a.arora@...cle.com>
Reviewed-by: Frederic Weisbecker <frederic@...nel.org>
Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
---
 kernel/rcu/tree_plugin.h | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index b025e95ede053..f444e8d3f68b4 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -984,13 +984,16 @@ static void rcu_preempt_check_blocked_tasks(struct rcu_node *rnp)
  */
 static void rcu_flavor_sched_clock_irq(int user)
 {
-	if (user || rcu_is_cpu_rrupt_from_idle()) {
+	if (user || rcu_is_cpu_rrupt_from_idle() ||
+	     (IS_ENABLED(CONFIG_PREEMPT_COUNT) &&
+	      (preempt_count() == HARDIRQ_OFFSET))) {
 
 		/*
 		 * Get here if this CPU took its interrupt from user
-		 * mode or from the idle loop, and if this is not a
-		 * nested interrupt.  In this case, the CPU is in
-		 * a quiescent state, so note it.
+		 * mode, from the idle loop without this being a nested
+		 * interrupt, or while not holding the task preempt count
+		 * (with PREEMPT_COUNT=y). In this case, the CPU is in a
+		 * quiescent state, so note it.
 		 *
 		 * No memory barrier is required here because rcu_qs()
 		 * references only CPU-local variables that other CPUs
-- 
2.40.1