[PATCH v4 2/2] locking/qrwlock: Don't contend with readers when setting _QW_WAITING

Waiman Long
2015-06-18T02:21:05Z

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.

Signed-off-by: Waiman Long <Waiman.Long@hp.com>
---
kernel/locking/qrwlock.c | 28 ++++++++++++++++++++++++----
1 files changed, 24 insertions(+), 4 deletions(-)

diff --git a/kernel/locking/qrwlock.c b/kernel/locking/qrwlock.c
index 7fd223c..38ad7e0 100644
--- a/kernel/locking/qrwlock.c
+++ b/kernel/locking/qrwlock.c
@@ -22,6 +22,26 @@
#include <linux/hardirq.h>
#include <asm/qrwlock.h>

+/*
+ * This internal data structure is used for optimizing access to some of
+ * the subfields within the atomic_t cnts.
+ */
+struct __qrwlock {
+ union {
+ atomic_t cnts;
+ struct {
+#ifdef __LITTLE_ENDIAN
+ u8 wmode; /* Writer mode */
+ u8 rcnts[3]; /* Reader counts */
+#else
+ u8 rcnts[3]; /* Reader counts */
+ u8 wmode; /* Writer mode */
+#endif
+ };
+ };
+ arch_spinlock_t lock;
+};
+
/**
* rspin_until_writer_unlock - inc reader count & spin until writer is gone
* @lock : Pointer to queue rwlock structure
@@ -108,10 +128,10 @@ void queue_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))
+ struct __qrwlock *l = (struct __qrwlock *)lock;
+
+ if (!READ_ONCE(l->wmode) &&
+ (cmpxchg(&l->wmode, 0, _QW_WAITING) == 0))
break;

cpu_relax_lowlatency();
--
1.7.1