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authorWaiman Long <Waiman.Long@hp.com>2015-06-09 11:19:13 -0400
committerIngo Molnar <mingo@kernel.org>2015-06-19 09:45:38 +0200
commit405963b6a57c60040bc1dad2597f7f4b897954d1 (patch)
tree960acc0117e6c8960456ed3227bc313a504d5b32 /kernel
parent92ae18371cb1abb4e186dd9d48de2bb0d9bba626 (diff)
locking/qrwlock: Don't contend with readers when setting _QW_WAITING
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> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Cc: Andrew Morton <akpm@linux-foundation.org> Cc: Arnd Bergmann <arnd@arndb.de> Cc: Borislav Petkov <bp@alien8.de> Cc: Douglas Hatch <doug.hatch@hp.com> Cc: H. Peter Anvin <hpa@zytor.com> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Scott J Norton <scott.norton@hp.com> Cc: Thomas Gleixner <tglx@linutronix.de> Link: http://lkml.kernel.org/r/1433863153-30722-3-git-send-email-Waiman.Long@hp.com Signed-off-by: Ingo Molnar <mingo@kernel.org>
Diffstat (limited to 'kernel')
-rw-r--r--kernel/locking/qrwlock.c28
1 files changed, 24 insertions, 4 deletions
diff --git a/kernel/locking/qrwlock.c b/kernel/locking/qrwlock.c
index 00c12bb390b5..6c5da483966b 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
@@ -107,10 +127,10 @@ 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();