diff mbox series

[rcu,06/11] osnoise: provide quiescent states

Message ID 20250225035516.26443-7-boqun.feng@gmail.com (mailing list archive)
State Handled Elsewhere
Headers show
Series Lazy Preempt changes for v6.15 | expand

Commit Message

Boqun Feng Feb. 25, 2025, 3:55 a.m. UTC
From: Ankur Arora <ankur.a.arora@oracle.com>

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>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
---
 kernel/trace/trace_osnoise.c | 32 +++++++++++++++-----------------
 1 file changed, 15 insertions(+), 17 deletions(-)

Comments

Steven Rostedt March 4, 2025, 7:21 p.m. UTC | #1
On Mon, 24 Feb 2025 19:55:11 -0800
Boqun Feng <boqun.feng@gmail.com> wrote:

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

Acked-by: Steven Rostedt (Google) <rostedt@goodmis.org>

-- Steve

> 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>
> Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
> Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
> ---
Boqun Feng March 5, 2025, 2:54 a.m. UTC | #2
On Tue, Mar 04, 2025 at 02:21:27PM -0500, Steven Rostedt wrote:
> On Mon, 24 Feb 2025 19:55:11 -0800
> Boqun Feng <boqun.feng@gmail.com> wrote:
> 
> > From: Ankur Arora <ankur.a.arora@oracle.com>
> > 
> > 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>
> 
> Acked-by: Steven Rostedt (Google) <rostedt@goodmis.org>
> 

Applied, thanks!

Regards,
Boqun

> -- Steve
> 
> > 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>
> > Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
> > Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
> > ---
diff mbox series

Patch

diff --git a/kernel/trace/trace_osnoise.c b/kernel/trace/trace_osnoise.c
index f3a2722ee4c0..512034e365ad 100644
--- a/kernel/trace/trace_osnoise.c
+++ b/kernel/trace/trace_osnoise.c
@@ -1542,27 +1542,25 @@  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