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Subject[PATCH tip/core/rcu 05/18] rcu/nocb: Check for deferred nocb wakeups before nohz_full early exit
Date
In theory, a timer is used to defer wakeups of no-CBs grace-period
kthreads when the wakeup cannot be done safely directly from the
call_rcu(). In practice, the one-jiffy delay is not always consistent
with timely callback invocation under heavy call_rcu() loads. Therefore,
there are a number of checks for a pending deferred wakeup, including
from the scheduling-clock interrupt. Unfortunately, this check follows
the rcu_nohz_full_cpu() early exit, which renders it useless on such CPUs.

This commit therefore moves the check for the pending deferred no-CB
wakeup to precede the rcu_nohz_full_cpu() early exit.

Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
---
kernel/rcu/tree.c | 8 ++++----
1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index ea479d81da7f..f1a25d17e3a0 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -2739,6 +2739,10 @@ static int rcu_pending(void)
/* Check for CPU stalls, if enabled. */
check_cpu_stall(rdp);

+ /* Does this CPU need a deferred NOCB wakeup? */
+ if (rcu_nocb_need_deferred_wakeup(rdp))
+ return 1;
+
/* Is this CPU a NO_HZ_FULL CPU that should ignore RCU? */
if (rcu_nohz_full_cpu())
return 0;
@@ -2763,10 +2767,6 @@ static int rcu_pending(void)
unlikely(READ_ONCE(rdp->gpwrap))) /* outside lock */
return 1;

- /* Does this CPU need a deferred NOCB wakeup? */
- if (rcu_nocb_need_deferred_wakeup(rdp))
- return 1;
-
/* nothing to do */
return 0;
}
--
2.17.1
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