linux-kernel - [PATCH] rcu: don't call rcu_preempt_note_context_switch() in rcu_check

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Date:	Mon, 29 Mar 2010 10:47:59 +0800
From:	Lai Jiangshan <laijs@...fujitsu.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...e.hu>,
	LKML <linux-kernel@...r.kernel.org>
Subject: [PATCH] rcu: don't call rcu_preempt_note_context_switch() in rcu_check_callbacks()


Even though in user mode or idle mode, rcu_check_callbacks() is not
context switch, so we don't call rcu_preempt_note_context_switch()
in rcu_check_callbacks().

Though there is no harm that calls rcu_preempt_note_context_switch()
in rcu_check_callbacks(), but it is waste.

rcu_check_callbacks()
  rcu_sched_qs()
    rcu_preempt_note_context_switch()
       Now, ->rcu_read_lock_nesting == 0, so we just calls
       rcu_preempt_qs(), but, rcu_preempt_check_callbacks()
       will call it again and set the ->rcu_read_unlock_special
       correct again.

So let rcu_preempt_check_callbacks() handle things for us.

Signed-off-by: Lai Jiangshan <laijs@...fujitsu.com>
---
diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index 3ec8160..c7847ba 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -95,7 +95,7 @@ static int rcu_gp_in_progress(struct rcu_state *rsp)
  * how many quiescent states passed, just if there was at least
  * one since the start of the grace period, this just sets a flag.
  */
-void rcu_sched_qs(int cpu)
+static void __rcu_sched_qs(int cpu)
 {
 	struct rcu_data *rdp;
 
@@ -103,6 +103,11 @@ void rcu_sched_qs(int cpu)
 	rdp->passed_quiesc_completed = rdp->gpnum - 1;
 	barrier();
 	rdp->passed_quiesc = 1;
+}
+
+void rcu_sched_qs(int cpu)
+{
+	__rcu_sched_qs(cpu);
 	rcu_preempt_note_context_switch(cpu);
 }
 
@@ -1138,12 +1143,12 @@ void rcu_check_callbacks(int cpu, int user)
 		 * a quiescent state, so note it.
 		 *
 		 * No memory barrier is required here because both
-		 * rcu_sched_qs() and rcu_bh_qs() reference only CPU-local
+		 * __rcu_sched_qs() and rcu_bh_qs() reference only CPU-local
 		 * variables that other CPUs neither access nor modify,
 		 * at least not while the corresponding CPU is online.
 		 */
 
-		rcu_sched_qs(cpu);
+		__rcu_sched_qs(cpu);
 		rcu_bh_qs(cpu);
 
 	} else if (!in_softirq()) {

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