linux-kernel - [PATCH 3.12 067/101] rcu: Use rcu_gp_kthread_wake() to wake up grace period kthreads

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Message-Id: <4ffc1f8b153b3b0e322fbac381ed4c240d1b7106.1417605922.git.jslaby@suse.cz>
Date:	Wed,  3 Dec 2014 12:26:29 +0100
From:	Jiri Slaby <jslaby@...e.cz>
To:	stable@...r.kernel.org
Cc:	linux-kernel@...r.kernel.org,
	Pranith Kumar <bobby.prani@...il.com>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Kamal Mostafa <kamal@...onical.com>,
	Jiri Slaby <jslaby@...e.cz>
Subject: [PATCH 3.12 067/101] rcu: Use rcu_gp_kthread_wake() to wake up grace period kthreads

From: Pranith Kumar <bobby.prani@...il.com>

3.12-stable review patch.  If anyone has any objections, please let me know.

===============

commit 2aa792e6faf1a00f5accf1f69e87e11a390ba2cd upstream.

The rcu_gp_kthread_wake() function checks for three conditions before
waking up grace period kthreads:

*  Is the thread we are trying to wake up the current thread?
*  Are the gp_flags zero? (all threads wait on non-zero gp_flags condition)
*  Is there no thread created for this flavour, hence nothing to wake up?

If any one of these condition is true, we do not call wake_up().
It was found that there are quite a few avoidable wake ups both during
idle time and under stress induced by rcutorture.

Idle:

Total:66000, unnecessary:66000, case1:61827, case2:66000, case3:0
Total:68000, unnecessary:68000, case1:63696, case2:68000, case3:0

rcutorture:

Total:254000, unnecessary:254000, case1:199913, case2:254000, case3:0
Total:256000, unnecessary:256000, case1:201784, case2:256000, case3:0

Here case{1-3} are the cases listed above. We can avoid these wake
ups by using rcu_gp_kthread_wake() to conditionally wake up the grace
period kthreads.

There is a comment about an implied barrier supplied by the wake_up()
logic.  This barrier is necessary for the awakened thread to see the
updated ->gp_flags.  This flag is always being updated with the root node
lock held. Also, the awakened thread tries to acquire the root node lock
before reading ->gp_flags because of which there is proper ordering.

Hence this commit tries to avoid calling wake_up() whenever we can by
using rcu_gp_kthread_wake() function.

Signed-off-by: Pranith Kumar <bobby.prani@...il.com>
CC: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
Cc: Kamal Mostafa <kamal@...onical.com>
Signed-off-by: Jiri Slaby <jslaby@...e.cz>
---
 kernel/rcutree.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index ec5848e9d5d9..e27526232b5f 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -1618,7 +1618,7 @@ static void rcu_report_qs_rsp(struct rcu_state *rsp, unsigned long flags)
 {
 	WARN_ON_ONCE(!rcu_gp_in_progress(rsp));
 	raw_spin_unlock_irqrestore(&rcu_get_root(rsp)->lock, flags);
-	wake_up(&rsp->gp_wq);  /* Memory barrier implied by wake_up() path. */
+	rcu_gp_kthread_wake(rsp);
 }

 /*
@@ -2188,7 +2188,7 @@ static void force_quiescent_state(struct rcu_state *rsp)
 	}
 	rsp->gp_flags |= RCU_GP_FLAG_FQS;
 	raw_spin_unlock_irqrestore(&rnp_old->lock, flags);
-	wake_up(&rsp->gp_wq);  /* Memory barrier implied by wake_up() path. */
+	rcu_gp_kthread_wake(rsp);
 }

 /*
-- 
2.1.3

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