linux-kernel - [PATCH v3 6/7] osnoise: provide quiescent states

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Message-Id: <20241213040658.2282640-7-ankur.a.arora@oracle.com>
Date: Thu, 12 Dec 2024 20:06:57 -0800
From: Ankur Arora <ankur.a.arora@...cle.com>
To: linux-kernel@...r.kernel.org
Cc: peterz@...radead.org, tglx@...utronix.de, paulmck@...nel.org,
        mingo@...nel.org, bigeasy@...utronix.de, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
        vschneid@...hat.com, frederic@...nel.org, ankur.a.arora@...cle.com,
        efault@....de, sshegde@...ux.ibm.com, boris.ostrovsky@...cle.com,
        Daniel Bristot de Oliveira <bristot@...nel.org>
Subject: [PATCH v3 6/7] osnoise: provide quiescent states

To reduce RCU noise for nohz_full configurations, osnoise depends
on cond_resched() providing quiescent states for PREEMPT_RCU=n
configurations. For PREEMPT_RCU=y configurations -- where
cond_resched() is a stub -- we do this by directly calling
rcu_momentary_eqs().

With (PREEMPT_LAZY=y, PREEMPT_DYNAMIC=n), however, we have a
configuration with (PREEMPTION=y, PREEMPT_RCU=n) where neither
of the above can help.

Handle that by providing an explicit quiescent state here for all
configurations.

As mentioned above this is not needed for non-stubbed cond_resched(),
but, providing a quiescent state here just pulls in one that a future
cond_resched() would provide, so doesn't cause any extra work for
this configuration.

Cc: Paul E. McKenney <paulmck@...nel.org>
Cc: Daniel Bristot de Oliveira <bristot@...nel.org>
Cc: Steven Rostedt <rostedt@...dmis.org>
Suggested-by: Paul E. McKenney <paulmck@...nel.org>
Acked-by: Daniel Bristot de Oliveira <bristot@...nel.org>
Signed-off-by: Ankur Arora <ankur.a.arora@...cle.com>
---
 kernel/trace/trace_osnoise.c | 32 +++++++++++++++-----------------
 1 file changed, 15 insertions(+), 17 deletions(-)

diff --git a/kernel/trace/trace_osnoise.c b/kernel/trace/trace_osnoise.c
index b9f96c77527d..2340ffcefb9d 100644
--- a/kernel/trace/trace_osnoise.c
+++ b/kernel/trace/trace_osnoise.c
@@ -1531,27 +1531,25 @@ static int run_osnoise(void)
 
 		/*
 		 * In some cases, notably when running on a nohz_full CPU with
-		 * a stopped tick PREEMPT_RCU has no way to account for QSs.
-		 * This will eventually cause unwarranted noise as PREEMPT_RCU
-		 * will force preemption as the means of ending the current
-		 * grace period. We avoid this problem by calling
-		 * rcu_momentary_eqs(), which performs a zero duration
-		 * EQS allowing PREEMPT_RCU to end the current grace period.
-		 * This call shouldn't be wrapped inside an RCU critical
-		 * section.
+		 * a stopped tick PREEMPT_RCU or PREEMPT_LAZY have no way to
+		 * account for QSs. This will eventually cause unwarranted
+		 * noise as RCU forces preemption as the means of ending the
+		 * current grace period.  We avoid this by calling
+		 * rcu_momentary_eqs(), which performs a zero duration EQS
+		 * allowing RCU to end the current grace period. This call
+		 * shouldn't be wrapped inside an RCU critical section.
 		 *
-		 * Note that in non PREEMPT_RCU kernels QSs are handled through
-		 * cond_resched()
+		 * Normally QSs for other cases are handled through cond_resched().
+		 * For simplicity, however, we call rcu_momentary_eqs() for all
+		 * configurations here.
 		 */
-		if (IS_ENABLED(CONFIG_PREEMPT_RCU)) {
-			if (!disable_irq)
-				local_irq_disable();
+		if (!disable_irq)
+			local_irq_disable();
 
-			rcu_momentary_eqs();
+		rcu_momentary_eqs();
 
-			if (!disable_irq)
-				local_irq_enable();
-		}
+		if (!disable_irq)
+			local_irq_enable();
 
 		/*
 		 * For the non-preemptive kernel config: let threads runs, if
-- 
2.43.5