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authorWaiman Long <longman@redhat.com>2019-05-20 16:59:02 -0400
committerIngo Molnar <mingo@kernel.org>2019-06-17 12:27:56 +0200
commit64489e78004cb5623211c75790cac90bd25ff5e9 (patch)
tree22ce261f7a0dae0b3a8d4294a3d326485d36c83e /kernel/locking/rwsem.h
parent5c1ec49b60cdb31e51010f8a647f3189b774bddf (diff)
locking/rwsem: Implement a new locking scheme
The current way of using various reader, writer and waiting biases in the rwsem code are confusing and hard to understand. I have to reread the rwsem count guide in the rwsem-xadd.c file from time to time to remind myself how this whole thing works. It also makes the rwsem code harder to be optimized. To make rwsem more sane, a new locking scheme similar to the one in qrwlock is now being used. The atomic long count has the following bit definitions: Bit 0 - writer locked bit Bit 1 - waiters present bit Bits 2-7 - reserved for future extension Bits 8-X - reader count (24/56 bits) The cmpxchg instruction is now used to acquire the write lock. The read lock is still acquired with xadd instruction, so there is no change here. This scheme will allow up to 16M/64P active readers which should be more than enough. We can always use some more reserved bits if necessary. With that change, we can deterministically know if a rwsem has been write-locked. Looking at the count alone, however, one cannot determine for certain if a rwsem is owned by readers or not as the readers that set the reader count bits may be in the process of backing out. So we still need the reader-owned bit in the owner field to be sure. With a locking microbenchmark running on 5.1 based kernel, the total locking rates (in kops/s) of the benchmark on a 8-socket 120-core IvyBridge-EX system before and after the patch were as follows: Before Patch After Patch # of Threads wlock rlock wlock rlock ------------ ----- ----- ----- ----- 1 30,659 31,341 31,055 31,283 2 8,909 16,457 9,884 17,659 4 9,028 15,823 8,933 20,233 8 8,410 14,212 7,230 17,140 16 8,217 25,240 7,479 24,607 The locking rates of the benchmark on a Power8 system were as follows: Before Patch After Patch # of Threads wlock rlock wlock rlock ------------ ----- ----- ----- ----- 1 12,963 13,647 13,275 13,601 2 7,570 11,569 7,902 10,829 4 5,232 5,516 5,466 5,435 8 5,233 3,386 5,467 3,168 The locking rates of the benchmark on a 2-socket ARM64 system were as follows: Before Patch After Patch # of Threads wlock rlock wlock rlock ------------ ----- ----- ----- ----- 1 21,495 21,046 21,524 21,074 2 5,293 10,502 5,333 10,504 4 5,325 11,463 5,358 11,631 8 5,391 11,712 5,470 11,680 The performance are roughly the same before and after the patch. There are run-to-run variations in performance. Runs with higher variances usually have higher throughput. Signed-off-by: Waiman Long <longman@redhat.com> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Cc: Borislav Petkov <bp@alien8.de> Cc: Davidlohr Bueso <dave@stgolabs.net> Cc: H. Peter Anvin <hpa@zytor.com> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: Tim Chen <tim.c.chen@linux.intel.com> Cc: Will Deacon <will.deacon@arm.com> Cc: huang ying <huang.ying.caritas@gmail.com> Link: https://lkml.kernel.org/r/20190520205918.22251-4-longman@redhat.com Signed-off-by: Ingo Molnar <mingo@kernel.org>
Diffstat (limited to 'kernel/locking/rwsem.h')
-rw-r--r--kernel/locking/rwsem.h74
1 files changed, 36 insertions, 38 deletions
diff --git a/kernel/locking/rwsem.h b/kernel/locking/rwsem.h
index eb9c8534299b..499a9b2bda82 100644
--- a/kernel/locking/rwsem.h
+++ b/kernel/locking/rwsem.h
@@ -42,24 +42,24 @@
#endif
/*
- * R/W semaphores originally for PPC using the stuff in lib/rwsem.c.
- * Adapted largely from include/asm-i386/rwsem.h
- * by Paul Mackerras <paulus@samba.org>.
- */
-
-/*
- * the semaphore definition
+ * The definition of the atomic counter in the semaphore:
+ *
+ * Bit 0 - writer locked bit
+ * Bit 1 - waiters present bit
+ * Bits 2-7 - reserved
+ * Bits 8-X - 24-bit (32-bit) or 56-bit reader count
+ *
+ * atomic_long_fetch_add() is used to obtain reader lock, whereas
+ * atomic_long_cmpxchg() will be used to obtain writer lock.
*/
-#ifdef CONFIG_64BIT
-# define RWSEM_ACTIVE_MASK 0xffffffffL
-#else
-# define RWSEM_ACTIVE_MASK 0x0000ffffL
-#endif
-
-#define RWSEM_ACTIVE_BIAS 0x00000001L
-#define RWSEM_WAITING_BIAS (-RWSEM_ACTIVE_MASK-1)
-#define RWSEM_ACTIVE_READ_BIAS RWSEM_ACTIVE_BIAS
-#define RWSEM_ACTIVE_WRITE_BIAS (RWSEM_WAITING_BIAS + RWSEM_ACTIVE_BIAS)
+#define RWSEM_WRITER_LOCKED (1UL << 0)
+#define RWSEM_FLAG_WAITERS (1UL << 1)
+#define RWSEM_READER_SHIFT 8
+#define RWSEM_READER_BIAS (1UL << RWSEM_READER_SHIFT)
+#define RWSEM_READER_MASK (~(RWSEM_READER_BIAS - 1))
+#define RWSEM_WRITER_MASK RWSEM_WRITER_LOCKED
+#define RWSEM_LOCK_MASK (RWSEM_WRITER_MASK|RWSEM_READER_MASK)
+#define RWSEM_READ_FAILED_MASK (RWSEM_WRITER_MASK|RWSEM_FLAG_WAITERS)
/*
* All writes to owner are protected by WRITE_ONCE() to make sure that
@@ -151,7 +151,8 @@ extern struct rw_semaphore *rwsem_downgrade_wake(struct rw_semaphore *sem);
*/
static inline void __down_read(struct rw_semaphore *sem)
{
- if (unlikely(atomic_long_inc_return_acquire(&sem->count) <= 0)) {
+ if (unlikely(atomic_long_fetch_add_acquire(RWSEM_READER_BIAS,
+ &sem->count) & RWSEM_READ_FAILED_MASK)) {
rwsem_down_read_failed(sem);
DEBUG_RWSEMS_WARN_ON(!((unsigned long)sem->owner &
RWSEM_READER_OWNED), sem);
@@ -162,7 +163,8 @@ static inline void __down_read(struct rw_semaphore *sem)
static inline int __down_read_killable(struct rw_semaphore *sem)
{
- if (unlikely(atomic_long_inc_return_acquire(&sem->count) <= 0)) {
+ if (unlikely(atomic_long_fetch_add_acquire(RWSEM_READER_BIAS,
+ &sem->count) & RWSEM_READ_FAILED_MASK)) {
if (IS_ERR(rwsem_down_read_failed_killable(sem)))
return -EINTR;
DEBUG_RWSEMS_WARN_ON(!((unsigned long)sem->owner &
@@ -183,11 +185,11 @@ static inline int __down_read_trylock(struct rw_semaphore *sem)
lockevent_inc(rwsem_rtrylock);
do {
if (atomic_long_try_cmpxchg_acquire(&sem->count, &tmp,
- tmp + RWSEM_ACTIVE_READ_BIAS)) {
+ tmp + RWSEM_READER_BIAS)) {
rwsem_set_reader_owned(sem);
return 1;
}
- } while (tmp >= 0);
+ } while (!(tmp & RWSEM_READ_FAILED_MASK));
return 0;
}
@@ -196,22 +198,16 @@ static inline int __down_read_trylock(struct rw_semaphore *sem)
*/
static inline void __down_write(struct rw_semaphore *sem)
{
- long tmp;
-
- tmp = atomic_long_add_return_acquire(RWSEM_ACTIVE_WRITE_BIAS,
- &sem->count);
- if (unlikely(tmp != RWSEM_ACTIVE_WRITE_BIAS))
+ if (unlikely(atomic_long_cmpxchg_acquire(&sem->count, 0,
+ RWSEM_WRITER_LOCKED)))
rwsem_down_write_failed(sem);
rwsem_set_owner(sem);
}
static inline int __down_write_killable(struct rw_semaphore *sem)
{
- long tmp;
-
- tmp = atomic_long_add_return_acquire(RWSEM_ACTIVE_WRITE_BIAS,
- &sem->count);
- if (unlikely(tmp != RWSEM_ACTIVE_WRITE_BIAS))
+ if (unlikely(atomic_long_cmpxchg_acquire(&sem->count, 0,
+ RWSEM_WRITER_LOCKED)))
if (IS_ERR(rwsem_down_write_failed_killable(sem)))
return -EINTR;
rwsem_set_owner(sem);
@@ -224,7 +220,7 @@ static inline int __down_write_trylock(struct rw_semaphore *sem)
lockevent_inc(rwsem_wtrylock);
tmp = atomic_long_cmpxchg_acquire(&sem->count, RWSEM_UNLOCKED_VALUE,
- RWSEM_ACTIVE_WRITE_BIAS);
+ RWSEM_WRITER_LOCKED);
if (tmp == RWSEM_UNLOCKED_VALUE) {
rwsem_set_owner(sem);
return true;
@@ -242,8 +238,9 @@ static inline void __up_read(struct rw_semaphore *sem)
DEBUG_RWSEMS_WARN_ON(!((unsigned long)sem->owner & RWSEM_READER_OWNED),
sem);
rwsem_clear_reader_owned(sem);
- tmp = atomic_long_dec_return_release(&sem->count);
- if (unlikely(tmp < -1 && (tmp & RWSEM_ACTIVE_MASK) == 0))
+ tmp = atomic_long_add_return_release(-RWSEM_READER_BIAS, &sem->count);
+ if (unlikely((tmp & (RWSEM_LOCK_MASK|RWSEM_FLAG_WAITERS))
+ == RWSEM_FLAG_WAITERS))
rwsem_wake(sem);
}
@@ -254,8 +251,8 @@ static inline void __up_write(struct rw_semaphore *sem)
{
DEBUG_RWSEMS_WARN_ON(sem->owner != current, sem);
rwsem_clear_owner(sem);
- if (unlikely(atomic_long_sub_return_release(RWSEM_ACTIVE_WRITE_BIAS,
- &sem->count) < 0))
+ if (unlikely(atomic_long_fetch_add_release(-RWSEM_WRITER_LOCKED,
+ &sem->count) & RWSEM_FLAG_WAITERS))
rwsem_wake(sem);
}
@@ -274,8 +271,9 @@ static inline void __downgrade_write(struct rw_semaphore *sem)
* write side. As such, rely on RELEASE semantics.
*/
DEBUG_RWSEMS_WARN_ON(sem->owner != current, sem);
- tmp = atomic_long_add_return_release(-RWSEM_WAITING_BIAS, &sem->count);
+ tmp = atomic_long_fetch_add_release(
+ -RWSEM_WRITER_LOCKED+RWSEM_READER_BIAS, &sem->count);
rwsem_set_reader_owned(sem);
- if (tmp < 0)
+ if (tmp & RWSEM_FLAG_WAITERS)
rwsem_downgrade_wake(sem);
}