[tip:locking/core] locking/qrwlock: Reduce reader/ writer to reader lock transfer latency

[tip-bot for Waiman Long]

Commit-ID:  ffffeaf318bd8da036eb8eb784b025a9f829201b
Gitweb:     http://git.kernel.org/tip/ffffeaf318bd8da036eb8eb784b025a9f829201b
Author:     Waiman Long <Waiman.Long@xxxxxx>
AuthorDate: Thu, 9 Jul 2015 12:32:22 -0400
Committer:  Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Mon, 3 Aug 2015 10:57:10 +0200

locking/qrwlock: Reduce reader/writer to reader lock transfer latency

Currently, a reader will check first to make sure that the writer mode
byte is cleared before incrementing the reader count. That waiting is
not really necessary. It increases the latency in the reader/writer
to reader transition and reduces readers performance.

This patch eliminates that waiting. It also has the side effect
of reducing the chance of writer lock stealing and improving the
fairness of the lock. Using a locking microbenchmark, a 10-threads 5M
locking loop of mostly readers (RW ratio = 10,000:1) has the following
performance numbers in a Haswell-EX box:

        Kernel          Locking Rate (Kops/s)
        ------          ---------------------
        4.1.1               15,063,081
        4.1.1+patch         17,241,552  (+14.4%)

Signed-off-by: Waiman Long <Waiman.Long@xxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Arnd Bergmann <arnd@xxxxxxxx>
Cc: Douglas Hatch <doug.hatch@xxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Scott J Norton <scott.norton@xxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Will Deacon <will.deacon@xxxxxxx>
Link: http://lkml.kernel.org/r/1436459543-29126-2-git-send-email-Waiman.Long@xxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
---
 kernel/locking/qrwlock.c | 12 ++++--------
 1 file changed, 4 insertions(+), 8 deletions(-)

diff --git a/kernel/locking/qrwlock.c b/kernel/locking/qrwlock.c
index d9c36c5..6a7a3b8 100644
--- a/kernel/locking/qrwlock.c
+++ b/kernel/locking/qrwlock.c
@@ -88,15 +88,11 @@ void queued_read_lock_slowpath(struct qrwlock *lock, u32 cnts)
 	arch_spin_lock(&lock->lock);
 
 	/*
-	 * At the head of the wait queue now, wait until the writer state
-	 * goes to 0 and then try to increment the reader count and get
-	 * the lock. It is possible that an incoming writer may steal the
-	 * lock in the interim, so it is necessary to check the writer byte
-	 * to make sure that the write lock isn't taken.
+	 * At the head of the wait queue now, increment the reader count
+	 * and wait until the writer, if it has the lock, has gone away.
+	 * At ths stage, it is not possible for a writer to remain in the
+	 * waiting state (_QW_WAITING). So there won't be any deadlock.
 	 */
-	while (atomic_read(&lock->cnts) & _QW_WMASK)
-		cpu_relax_lowlatency();
-
 	cnts = atomic_add_return(_QR_BIAS, &lock->cnts) - _QR_BIAS;
 	rspin_until_writer_unlock(lock, cnts);
 
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