[PATCH v5 3/3] locking/qrwlock: Don't contend with readers when setting _QW_WAITING

Waiman Long
2015-06-19T15:51:38Z

The current cmpxchg() loop in setting the _QW_WAITING flag for writers
in queue_write_lock_slowpath() will contend with incoming readers
causing possibly extra cmpxchg() operations that are wasteful. This
patch changes the code to do a byte cmpxchg() to eliminate contention
with new readers.

A multithreaded microbenchmark running 5M read_lock/write_lock loop
on a 8-socket 80-core Westmere-EX machine running 4.0 based kernel
with the qspinlock patch have the following execution times (in ms)
with and without the patch:

With R:W ratio = 5:1

Threads w/o patch with patch % change
------- --------- ---------- --------
2 990 895 -9.6%
3 2136 1912 -10.5%
4 3166 2830 -10.6%
5 3953 3629 -8.2%
6 4628 4405 -4.8%
7 5344 5197 -2.8%
8 6065 6004 -1.0%
9 6826 6811 -0.2%
10 7599 7599 0.0%
15 9757 9766 +0.1%
20 13767 13817 +0.4%

With small number of contending threads, this patch can improve
locking performance by up to 10%. With more contending threads,
however, the gain diminishes.

With the extended qrwlock structure defined in asm-generic/qrwlock,
the queue_write_unlock() function is also simplified to a
smp_store_release() call.

Signed-off-by: Waiman Long <Waiman.Long@hp.com>
---
arch/x86/include/asm/qrwlock.h | 3 +--
include/asm-generic/qrwlock_types.h | 18 +++++++++++++++++-
kernel/locking/qrwlock.c | 6 ++----
3 files changed, 20 insertions(+), 7 deletions(-)

diff --git a/arch/x86/include/asm/qrwlock.h b/arch/x86/include/asm/qrwlock.h
index a8810bf..5678b0a 100644
--- a/arch/x86/include/asm/qrwlock.h
+++ b/arch/x86/include/asm/qrwlock.h
@@ -7,8 +7,7 @@
#define queued_write_unlock queued_write_unlock
static inline void queued_write_unlock(struct qrwlock *lock)
{
- barrier();
- ACCESS_ONCE(*(u8 *)&lock->cnts) = 0;
+ smp_store_release(&lock->wmode, 0);
}
#endif

diff --git a/include/asm-generic/qrwlock_types.h b/include/asm-generic/qrwlock_types.h
index 4d76f24..d614cde 100644
--- a/include/asm-generic/qrwlock_types.h
+++ b/include/asm-generic/qrwlock_types.h
@@ -3,13 +3,29 @@

#include <linux/types.h>
#include <asm/spinlock_types.h>
+#include <asm/byteorder.h>

/*
* The queue read/write lock data structure
+ *
+ * The 32-bit count is divided into an 8-bit writer mode byte
+ * (least significant byte) and a 24-bit reader count.
+ *
*/

typedef struct qrwlock {
- atomic_t cnts;
+ union {
+ atomic_t cnts;
+ struct {
+#ifdef __LITTLE_ENDIAN
+ u8 wmode; /* Writer mode */
+ u8 rcnt[3]; /* Reader count */
+#else
+ u8 rcnt[3]; /* Reader count */
+ u8 wmode; /* Writer mode */
+#endif
+ };
+ };
arch_spinlock_t lock;
} arch_rwlock_t;

diff --git a/kernel/locking/qrwlock.c b/kernel/locking/qrwlock.c
index 26ca0ca..a7ac2c5 100644
--- a/kernel/locking/qrwlock.c
+++ b/kernel/locking/qrwlock.c
@@ -108,10 +108,8 @@ void queued_write_lock_slowpath(struct qrwlock *lock)
* or wait for a previous writer to go away.
*/
for (;;) {
- cnts = atomic_read(&lock->cnts);
- if (!(cnts & _QW_WMASK) &&
- (atomic_cmpxchg(&lock->cnts, cnts,
- cnts | _QW_WAITING) == cnts))
+ if (!READ_ONCE(lock->wmode) &&
+ (cmpxchg(&lock->wmode, 0, _QW_WAITING) == 0))
break;

cpu_relax_lowlatency();
--
1.7.1