osnoise: provide quiescent states

JIRA: https://issues.redhat.com/browse/RHEL-117874

commit 9fd858c
Author: Ankur Arora <ankur.a.arora@oracle.com>
Date:   Thu Dec 12 20:06:57 2024 -0800

    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@kernel.org>
    Cc: Daniel Bristot de Oliveira <bristot@kernel.org>
    Cc: Steven Rostedt <rostedt@goodmis.org>
    Suggested-by: Paul E. McKenney <paulmck@kernel.org>
    Acked-by: Daniel Bristot de Oliveira <bristot@kernel.org>
    Signed-off-by: Ankur Arora <ankur.a.arora@oracle.com>
    Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
    Acked-by: Steven Rostedt (Google) <rostedt@goodmis.org>
    Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
    Signed-off-by: Boqun Feng <boqun.feng@gmail.com>

Signed-off-by: Tomas Glozar <tglozar@redhat.com>

Author: lenticularis39

kernel/trace/trace_osnoise.c

@@ -1525,27 +1525,25 @@ 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