Commit 9fd858cc authored by Ankur Arora's avatar Ankur Arora Committed by Boqun Feng
Browse files

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: default avatarPaul E. McKenney <paulmck@kernel.org>
Acked-by: default avatarDaniel Bristot de Oliveira <bristot@kernel.org>
Signed-off-by: default avatarAnkur Arora <ankur.a.arora@oracle.com>
Reviewed-by: default avatarFrederic Weisbecker <frederic@kernel.org>
Acked-by: default avatarSteven Rostedt (Google) <rostedt@goodmis.org>
Signed-off-by: default avatarPaul E. McKenney <paulmck@kernel.org>
Signed-off-by: default avatarBoqun Feng <boqun.feng@gmail.com>
parent 83b28cfe

kernel/trace/trace_osnoise.c

+15 −17
Original line number Diff line number Diff line
@@ -1542,19 +1542,18 @@ 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();
@@ -1562,7 +1561,6 @@ static int run_osnoise(void) 


		if (!disable_irq)

			local_irq_enable();

		}



		/*

		 * For the non-preemptive kernel config: let threads runs, if