From: Ankur Arora <ankur.a.arora@xxxxxxxxxx>
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@xxxxxxxxxx>
Cc: Daniel Bristot de Oliveira <bristot@xxxxxxxxxx>
Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
Suggested-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
Acked-by: Daniel Bristot de Oliveira <bristot@xxxxxxxxxx>
Signed-off-by: Ankur Arora <ankur.a.arora@xxxxxxxxxx>
Reviewed-by: Frederic Weisbecker <frederic@xxxxxxxxxx>
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
---
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 b9f96c77527db..2340ffcefb9d1 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
--
2.40.1
