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>

diff --git a/kernel/trace/trace_osnoise.c b/kernel/trace/trace_osnoise.c
index f3a2722ee4c07882e420c3c8cb4dafa85ef05467..512034e365ad29d5736e532f352168bca5c136b0 100644 (file)
--- 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