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 |
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> > ---
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 --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