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From: Tim Chen <tim.c.chen@linux.intel.com>
To: Ingo Molnar <mingo@elte.hu>, Andrew Morton <akpm@linux-foundation.org>
Cc: Andrea Arcangeli <aarcange@redhat.com>,
	Alex Shi <alex.shi@intel.com>, Andi Kleen <andi@firstfloor.org>,
	Michel Lespinasse <walken@google.com>,
	Davidlohr Bueso <davidlohr.bueso@hp.com>,
	Matthew R Wilcox <matthew.r.wilcox@intel.com>,
	Dave Hansen <dave.hansen@intel.com>,
	Peter Zijlstra <a.p.zijlstra@chello.nl>,
	Rik van Riel <riel@redhat.com>,
	Peter Hurley <peter@hurleysoftware.com>,
	Tim Chen <tim.c.chen@linux.intel.com>,
	linux-kernel@vger.kernel.org, linux-mm <linux-mm@kvack.org>
Subject: [PATCH v3 1/5] rwsem: check the lock before cpmxchg in down_write_trylock
Date: Wed, 26 Jun 2013 09:27:52 -0700	[thread overview]
Message-ID: <1372264072.22432.121.camel@schen9-DESK> (raw)
In-Reply-To: <cover.1372261602.git.tim.c.chen@linux.intel.com>

Cmpxchg will cause the cacheline bouning when do the value checking,
that cause scalability issue in a large machine (like a 80 core box).

So a lock pre-read can relief this contention.

Signed-off-by: Alex Shi <alex.shi@intel.com>
---
 include/asm-generic/rwsem.h |    8 ++++----
 1 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/include/asm-generic/rwsem.h b/include/asm-generic/rwsem.h
index bb1e2cd..5ba80e7 100644
--- a/include/asm-generic/rwsem.h
+++ b/include/asm-generic/rwsem.h
@@ -70,11 +70,11 @@ static inline void __down_write(struct rw_semaphore *sem)
 
 static inline int __down_write_trylock(struct rw_semaphore *sem)
 {
-	long tmp;
+	if (unlikely(sem->count != RWSEM_UNLOCKED_VALUE))
+		return 0;
 
-	tmp = cmpxchg(&sem->count, RWSEM_UNLOCKED_VALUE,
-		      RWSEM_ACTIVE_WRITE_BIAS);
-	return tmp == RWSEM_UNLOCKED_VALUE;
+	return cmpxchg(&sem->count, RWSEM_UNLOCKED_VALUE,
+		      RWSEM_ACTIVE_WRITE_BIAS) == RWSEM_UNLOCKED_VALUE;
 }
 
 /*
-- 
1.7.4.4




WARNING: multiple messages have this Message-ID (diff)
From: Tim Chen <tim.c.chen@linux.intel.com>
To: Ingo Molnar <mingo@elte.hu>, Andrew Morton <akpm@linux-foundation.org>
Cc: Andrea Arcangeli <aarcange@redhat.com>,
	Alex Shi <alex.shi@intel.com>, Andi Kleen <andi@firstfloor.org>,
	Michel Lespinasse <walken@google.com>,
	Davidlohr Bueso <davidlohr.bueso@hp.com>,
	Matthew R Wilcox <matthew.r.wilcox@intel.com>,
	Dave Hansen <dave.hansen@intel.com>,
	Peter Zijlstra <a.p.zijlstra@chello.nl>,
	Rik van Riel <riel@redhat.com>,
	Peter Hurley <peter@hurleysoftware.com>,
	Tim Chen <tim.c.chen@linux.intel.com>,
	linux-kernel@vger.kernel.org, linux-mm <linux-mm@kvack.org>
Subject: [PATCH v3 1/5] rwsem: check the lock before cpmxchg in down_write_trylock
Date: Wed, 26 Jun 2013 09:27:52 -0700	[thread overview]
Message-ID: <1372264072.22432.121.camel@schen9-DESK> (raw)
In-Reply-To: <cover.1372261602.git.tim.c.chen@linux.intel.com>

Cmpxchg will cause the cacheline bouning when do the value checking,
that cause scalability issue in a large machine (like a 80 core box).

So a lock pre-read can relief this contention.

Signed-off-by: Alex Shi <alex.shi@intel.com>
---
 include/asm-generic/rwsem.h |    8 ++++----
 1 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/include/asm-generic/rwsem.h b/include/asm-generic/rwsem.h
index bb1e2cd..5ba80e7 100644
--- a/include/asm-generic/rwsem.h
+++ b/include/asm-generic/rwsem.h
@@ -70,11 +70,11 @@ static inline void __down_write(struct rw_semaphore *sem)
 
 static inline int __down_write_trylock(struct rw_semaphore *sem)
 {
-	long tmp;
+	if (unlikely(sem->count != RWSEM_UNLOCKED_VALUE))
+		return 0;
 
-	tmp = cmpxchg(&sem->count, RWSEM_UNLOCKED_VALUE,
-		      RWSEM_ACTIVE_WRITE_BIAS);
-	return tmp == RWSEM_UNLOCKED_VALUE;
+	return cmpxchg(&sem->count, RWSEM_UNLOCKED_VALUE,
+		      RWSEM_ACTIVE_WRITE_BIAS) == RWSEM_UNLOCKED_VALUE;
 }
 
 /*
-- 
1.7.4.4



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       reply	other threads:[~2013-06-26 16:27 UTC|newest]

Thread overview: 10+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <cover.1372261602.git.tim.c.chen@linux.intel.com>
2013-06-26 16:27 ` Tim Chen [this message]
2013-06-26 16:27   ` [PATCH v3 1/5] rwsem: check the lock before cpmxchg in down_write_trylock Tim Chen
2013-06-26 16:27 ` [PATCH v3 2/5] rwsem: remove 'out' label in do_wake Tim Chen
2013-06-26 16:27   ` Tim Chen
2013-06-26 16:27 ` [PATCH v3 3/5] rwsem: remove try_reader_grant label do_wake Tim Chen
2013-06-26 16:27   ` Tim Chen
2013-06-26 16:28 ` [PATCH v3 4/5] rwsem/wake: check lock before do atomic update Tim Chen
2013-06-26 16:28   ` Tim Chen
2013-06-26 16:28 ` [PATCH v3 5/5] rwsem: do optimistic spinning for writer lock acquisition Tim Chen
2013-06-26 16:28   ` Tim Chen

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