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From: "Paul E. McKenney" <paulmck@linux.ibm.com>
To: rcu@vger.kernel.org
Cc: linux-kernel@vger.kernel.org, mingo@kernel.org,
	jiangshanlai@gmail.com, dipankar@in.ibm.com,
	akpm@linux-foundation.org, mathieu.desnoyers@efficios.com,
	josh@joshtriplett.org, tglx@linutronix.de, peterz@infradead.org,
	rostedt@goodmis.org, dhowells@redhat.com, edumazet@google.com,
	fweisbec@gmail.com, oleg@redhat.com, joel@joelfernandes.org,
	"Paul E. McKenney" <paulmck@linux.ibm.com>
Subject: [PATCH tip/core/rcu 05/18] rcu/nocb: Check for deferred nocb wakeups before nohz_full early exit
Date: Thu,  1 Aug 2019 16:07:57 -0700	[thread overview]
Message-ID: <20190801230810.21570-5-paulmck@linux.ibm.com> (raw)
In-Reply-To: <20190801230744.GA19115@linux.ibm.com>

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


  parent reply	other threads:[~2019-08-01 23:08 UTC|newest]

Thread overview: 19+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-08-01 23:07 [PATCH tip/core/rcu 0/18] No-CBs cblist updates for v5.3-rc2 Paul E. McKenney
2019-08-01 23:07 ` [PATCH tip/core/rcu 01/18] rcu/nocb: Use separate flag to indicate disabled ->cblist Paul E. McKenney
2019-08-01 23:07 ` [PATCH tip/core/rcu 02/18] rcu/nocb: Use separate flag to indicate offloaded ->cblist Paul E. McKenney
2019-08-01 23:07 ` [PATCH tip/core/rcu 03/18] rcu/nocb: Add checks for offloaded callback processing Paul E. McKenney
2019-08-01 23:07 ` [PATCH tip/core/rcu 04/18] rcu/nocb: Make rcutree_migrate_callbacks() start at leaf rcu_node structure Paul E. McKenney
2019-08-01 23:07 ` Paul E. McKenney [this message]
2019-08-01 23:07 ` [PATCH tip/core/rcu 06/18] rcu/nocb: Remove deferred wakeup checks for extended quiescent states Paul E. McKenney
2019-08-01 23:07 ` [PATCH tip/core/rcu 07/18] rcu/nocb: Allow lockless use of rcu_segcblist_restempty() Paul E. McKenney
2019-08-01 23:08 ` [PATCH tip/core/rcu 08/18] rcu/nocb: Allow lockless use of rcu_segcblist_empty() Paul E. McKenney
2019-08-01 23:08 ` [PATCH tip/core/rcu 09/18] rcu/nocb: Leave ->cblist enabled for no-CBs CPUs Paul E. McKenney
2019-08-01 23:08 ` [PATCH tip/core/rcu 10/18] rcu/nocb: Use rcu_segcblist " Paul E. McKenney
2019-08-01 23:08 ` [PATCH tip/core/rcu 11/18] rcu/nocb: Remove obsolete nocb_head and nocb_tail fields Paul E. McKenney
2019-08-01 23:08 ` [PATCH tip/core/rcu 12/18] rcu/nocb: Remove obsolete nocb_q_count and nocb_q_count_lazy fields Paul E. McKenney
2019-08-01 23:08 ` [PATCH tip/core/rcu 13/18] rcu/nocb: Remove obsolete nocb_cb_tail and nocb_cb_head fields Paul E. McKenney
2019-08-01 23:08 ` [PATCH tip/core/rcu 14/18] rcu/nocb: Remove obsolete nocb_gp_head and nocb_gp_tail fields Paul E. McKenney
2019-08-01 23:08 ` [PATCH tip/core/rcu 15/18] rcu/nocb: Use build-time no-CBs check in rcu_do_batch() Paul E. McKenney
2019-08-01 23:08 ` [PATCH tip/core/rcu 16/18] rcu/nocb: Use build-time no-CBs check in rcu_core() Paul E. McKenney
2019-08-01 23:08 ` [PATCH tip/core/rcu 17/18] rcu/nocb: Use build-time no-CBs check in rcu_pending() Paul E. McKenney
2019-08-01 23:08 ` [PATCH tip/core/rcu 18/18] rcu/nocb: Suppress uninitialized false-positive in nocb_gp_wait() Paul E. McKenney

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