From: Paul E. McKenney <paulmck@linux.ibm.com>
Date: Tue, 16 Apr 2019 22:15:24 +0000 (-0700)
Subject: rcu/nocb: Remove deferred wakeup checks for extended quiescent states
X-Git-Tag: for-5.4/post-2019-09-24~45^2~1^2^2~33
X-Git-Url: https://git.kernel.dk/?a=commitdiff_plain;h=ca5c8258081178c60b27e3532d9ea95b6eaa7040;p=linux-block.git

rcu/nocb: Remove deferred wakeup checks for extended quiescent states

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@linux.ibm.com>
---

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,
 				   -atomic_long_read(&rdp->nocb_q_count_lazy),
 				   -rcu_get_n_cbs_nocb_cpu(rdp));
 
-	/*
-	 * 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;
 }