[PATCH tip/core/rcu 06/18] rcu/nocb: Remove deferred wakeup checks for extended quiescent states

From: Paul E. McKenney
Date: Thu Aug 01 2019 - 19:08:30 EST

The idea behind the checks for extended quiescent states at the end of
__call_rcu_nocb() is to handle cases where call_rcu() is invoked directly
from within an extended quiescent state, for example, from the idle loop.
However, this will result in a timer-mediated deferred wakeup, which
will cause the needed wakeup to happen within a jiffy or thereabouts.
There should be no forward-progress concerns, and if there are, the proper
response is to exit the extended quiescent state while executing the
endless blast of call_rcu() invocations, for example, using RCU_NONIDLE().
Given the more realistic case of an isolated call_rcu() invocation, there
should be no problem.

This commit therefore removes the checks for invoking call_rcu() within
an extended quiescent state for on no-CBs CPUs.

Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxx>
kernel/rcu/tree_plugin.h | 10 ----------
1 file changed, 10 deletions(-)

diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index fc6133eed50a..9936a66b80bb 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -1709,16 +1709,6 @@ static bool __call_rcu_nocb(struct rcu_data *rdp, struct rcu_head *rhp,

- /*
- * If called from an extended quiescent state with interrupts
- * disabled, invoke the RCU core in order to allow the idle-entry
- * deferred-wakeup check to function.
- */
- if (irqs_disabled_flags(flags) &&
- !rcu_is_watching() &&
- cpu_online(smp_processor_id()))
- invoke_rcu_core();
return true;