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authorDavidlohr Bueso <davidlohr@hp.com>2014-05-02 22:24:15 +0400
committerIngo Molnar <mingo@kernel.org>2014-06-05 12:38:21 +0400
commit4fc828e24cd9c385d3a44e1b499ec7fc70239d8a (patch)
tree9b53c7b53d6d487e03a383486171279965d3af15 /include/linux/rwsem.h
parent3cf2f34e1a3d4d5ff209d087925cf950e52f4805 (diff)
downloadlinux-4fc828e24cd9c385d3a44e1b499ec7fc70239d8a.tar.xz
locking/rwsem: Support optimistic spinning
We have reached the point where our mutexes are quite fine tuned for a number of situations. This includes the use of heuristics and optimistic spinning, based on MCS locking techniques. Exclusive ownership of read-write semaphores are, conceptually, just about the same as mutexes, making them close cousins. To this end we need to make them both perform similarly, and right now, rwsems are simply not up to it. This was discovered by both reverting commit 4fc3f1d6 (mm/rmap, migration: Make rmap_walk_anon() and try_to_unmap_anon() more scalable) and similarly, converting some other mutexes (ie: i_mmap_mutex) to rwsems. This creates a situation where users have to choose between a rwsem and mutex taking into account this important performance difference. Specifically, biggest difference between both locks is when we fail to acquire a mutex in the fastpath, optimistic spinning comes in to play and we can avoid a large amount of unnecessary sleeping and overhead of moving tasks in and out of wait queue. Rwsems do not have such logic. This patch, based on the work from Tim Chen and I, adds support for write-side optimistic spinning when the lock is contended. It also includes support for the recently added cancelable MCS locking for adaptive spinning. Note that is is only applicable to the xadd method, and the spinlock rwsem variant remains intact. Allowing optimistic spinning before putting the writer on the wait queue reduces wait queue contention and provided greater chance for the rwsem to get acquired. With these changes, rwsem is on par with mutex. The performance benefits can be seen on a number of workloads. For instance, on a 8 socket, 80 core 64bit Westmere box, aim7 shows the following improvements in throughput: +--------------+---------------------+-----------------+ | Workload | throughput-increase | number of users | +--------------+---------------------+-----------------+ | alltests | 20% | >1000 | | custom | 27%, 60% | 10-100, >1000 | | high_systime | 36%, 30% | >100, >1000 | | shared | 58%, 29% | 10-100, >1000 | +--------------+---------------------+-----------------+ There was also improvement on smaller systems, such as a quad-core x86-64 laptop running a 30Gb PostgreSQL (pgbench) workload for up to +60% in throughput for over 50 clients. Additionally, benefits were also noticed in exim (mail server) workloads. Furthermore, no performance regression have been seen at all. Based-on-work-from: Tim Chen <tim.c.chen@linux.intel.com> Signed-off-by: Davidlohr Bueso <davidlohr@hp.com> [peterz: rej fixup due to comment patches, sched/rt.h header] Signed-off-by: Peter Zijlstra <peterz@infradead.org> Cc: Alex Shi <alex.shi@linaro.org> Cc: Andi Kleen <andi@firstfloor.org> Cc: Michel Lespinasse <walken@google.com> Cc: Rik van Riel <riel@redhat.com> Cc: Peter Hurley <peter@hurleysoftware.com> Cc: "Paul E.McKenney" <paulmck@linux.vnet.ibm.com> Cc: Jason Low <jason.low2@hp.com> Cc: Aswin Chandramouleeswaran <aswin@hp.com> Cc: Andrew Morton <akpm@linux-foundation.org> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: "Scott J Norton" <scott.norton@hp.com> Cc: Andrea Arcangeli <aarcange@redhat.com> Cc: Chris Mason <clm@fb.com> Cc: Josef Bacik <jbacik@fusionio.com> Link: http://lkml.kernel.org/r/1399055055.6275.15.camel@buesod1.americas.hpqcorp.net Signed-off-by: Ingo Molnar <mingo@kernel.org>
Diffstat (limited to 'include/linux/rwsem.h')
-rw-r--r--include/linux/rwsem.h25
1 files changed, 22 insertions, 3 deletions
diff --git a/include/linux/rwsem.h b/include/linux/rwsem.h
index 03f3b05e8ec1..3e108f154cb6 100644
--- a/include/linux/rwsem.h
+++ b/include/linux/rwsem.h
@@ -16,6 +16,7 @@
#include <linux/atomic.h>
+struct optimistic_spin_queue;
struct rw_semaphore;
#ifdef CONFIG_RWSEM_GENERIC_SPINLOCK
@@ -23,9 +24,17 @@ struct rw_semaphore;
#else
/* All arch specific implementations share the same struct */
struct rw_semaphore {
- long count;
- raw_spinlock_t wait_lock;
- struct list_head wait_list;
+ long count;
+ raw_spinlock_t wait_lock;
+ struct list_head wait_list;
+#ifdef CONFIG_SMP
+ /*
+ * Write owner. Used as a speculative check to see
+ * if the owner is running on the cpu.
+ */
+ struct task_struct *owner;
+ struct optimistic_spin_queue *osq; /* spinner MCS lock */
+#endif
#ifdef CONFIG_DEBUG_LOCK_ALLOC
struct lockdep_map dep_map;
#endif
@@ -55,11 +64,21 @@ static inline int rwsem_is_locked(struct rw_semaphore *sem)
# define __RWSEM_DEP_MAP_INIT(lockname)
#endif
+#ifdef CONFIG_SMP
+#define __RWSEM_INITIALIZER(name) \
+ { RWSEM_UNLOCKED_VALUE, \
+ __RAW_SPIN_LOCK_UNLOCKED(name.wait_lock), \
+ LIST_HEAD_INIT((name).wait_list), \
+ NULL, /* owner */ \
+ NULL /* mcs lock */ \
+ __RWSEM_DEP_MAP_INIT(name) }
+#else
#define __RWSEM_INITIALIZER(name) \
{ RWSEM_UNLOCKED_VALUE, \
__RAW_SPIN_LOCK_UNLOCKED(name.wait_lock), \
LIST_HEAD_INIT((name).wait_list) \
__RWSEM_DEP_MAP_INIT(name) }
+#endif
#define DECLARE_RWSEM(name) \
struct rw_semaphore name = __RWSEM_INITIALIZER(name)