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authorPeter Zijlstra <peterz@infradead.org>2016-07-14 21:08:46 +0300
committerIngo Molnar <mingo@kernel.org>2016-08-10 15:34:01 +0300
commit80127a39681bd68c959f0953f84a830cbd7c3b1c (patch)
tree223bcc2a5cbec5c0873f8fae85a98797f94e6c56 /include/linux/percpu-rwsem.h
parent08be8f63c40c030b5cf95b4368e314e563a86301 (diff)
downloadlinux-80127a39681bd68c959f0953f84a830cbd7c3b1c.tar.xz
locking/percpu-rwsem: Optimize readers and reduce global impact
Currently the percpu-rwsem switches to (global) atomic ops while a writer is waiting; which could be quite a while and slows down releasing the readers. This patch cures this problem by ordering the reader-state vs reader-count (see the comments in __percpu_down_read() and percpu_down_write()). This changes a global atomic op into a full memory barrier, which doesn't have the global cacheline contention. This also enables using the percpu-rwsem with rcu_sync disabled in order to bias the implementation differently, reducing the writer latency by adding some cost to readers. Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Reviewed-by: Oleg Nesterov <oleg@redhat.com> Cc: Andrew Morton <akpm@linux-foundation.org> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: Paul McKenney <paulmck@linux.vnet.ibm.com> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: linux-kernel@vger.kernel.org [ Fixed modular build. ] Signed-off-by: Ingo Molnar <mingo@kernel.org>
Diffstat (limited to 'include/linux/percpu-rwsem.h')
-rw-r--r--include/linux/percpu-rwsem.h84
1 files changed, 75 insertions, 9 deletions
diff --git a/include/linux/percpu-rwsem.h b/include/linux/percpu-rwsem.h
index c2fa3ecb0dce..146efefde2a1 100644
--- a/include/linux/percpu-rwsem.h
+++ b/include/linux/percpu-rwsem.h
@@ -10,30 +10,96 @@
struct percpu_rw_semaphore {
struct rcu_sync rss;
- unsigned int __percpu *fast_read_ctr;
+ unsigned int __percpu *read_count;
struct rw_semaphore rw_sem;
- atomic_t slow_read_ctr;
- wait_queue_head_t write_waitq;
+ wait_queue_head_t writer;
+ int readers_block;
};
-extern void percpu_down_read(struct percpu_rw_semaphore *);
-extern int percpu_down_read_trylock(struct percpu_rw_semaphore *);
-extern void percpu_up_read(struct percpu_rw_semaphore *);
+extern int __percpu_down_read(struct percpu_rw_semaphore *, int);
+extern void __percpu_up_read(struct percpu_rw_semaphore *);
+
+static inline void percpu_down_read(struct percpu_rw_semaphore *sem)
+{
+ might_sleep();
+
+ rwsem_acquire_read(&sem->rw_sem.dep_map, 0, 0, _RET_IP_);
+
+ preempt_disable();
+ /*
+ * We are in an RCU-sched read-side critical section, so the writer
+ * cannot both change sem->state from readers_fast and start checking
+ * counters while we are here. So if we see !sem->state, we know that
+ * the writer won't be checking until we're past the preempt_enable()
+ * and that one the synchronize_sched() is done, the writer will see
+ * anything we did within this RCU-sched read-size critical section.
+ */
+ __this_cpu_inc(*sem->read_count);
+ if (unlikely(!rcu_sync_is_idle(&sem->rss)))
+ __percpu_down_read(sem, false); /* Unconditional memory barrier */
+ preempt_enable();
+ /*
+ * The barrier() from preempt_enable() prevents the compiler from
+ * bleeding the critical section out.
+ */
+}
+
+static inline int percpu_down_read_trylock(struct percpu_rw_semaphore *sem)
+{
+ int ret = 1;
+
+ preempt_disable();
+ /*
+ * Same as in percpu_down_read().
+ */
+ __this_cpu_inc(*sem->read_count);
+ if (unlikely(!rcu_sync_is_idle(&sem->rss)))
+ ret = __percpu_down_read(sem, true); /* Unconditional memory barrier */
+ preempt_enable();
+ /*
+ * The barrier() from preempt_enable() prevents the compiler from
+ * bleeding the critical section out.
+ */
+
+ if (ret)
+ rwsem_acquire_read(&sem->rw_sem.dep_map, 0, 1, _RET_IP_);
+
+ return ret;
+}
+
+static inline void percpu_up_read(struct percpu_rw_semaphore *sem)
+{
+ /*
+ * The barrier() in preempt_disable() prevents the compiler from
+ * bleeding the critical section out.
+ */
+ preempt_disable();
+ /*
+ * Same as in percpu_down_read().
+ */
+ if (likely(rcu_sync_is_idle(&sem->rss)))
+ __this_cpu_dec(*sem->read_count);
+ else
+ __percpu_up_read(sem); /* Unconditional memory barrier */
+ preempt_enable();
+
+ rwsem_release(&sem->rw_sem.dep_map, 1, _RET_IP_);
+}
extern void percpu_down_write(struct percpu_rw_semaphore *);
extern void percpu_up_write(struct percpu_rw_semaphore *);
extern int __percpu_init_rwsem(struct percpu_rw_semaphore *,
const char *, struct lock_class_key *);
+
extern void percpu_free_rwsem(struct percpu_rw_semaphore *);
-#define percpu_init_rwsem(brw) \
+#define percpu_init_rwsem(sem) \
({ \
static struct lock_class_key rwsem_key; \
- __percpu_init_rwsem(brw, #brw, &rwsem_key); \
+ __percpu_init_rwsem(sem, #sem, &rwsem_key); \
})
-
#define percpu_rwsem_is_held(sem) lockdep_is_held(&(sem)->rw_sem)
static inline void percpu_rwsem_release(struct percpu_rw_semaphore *sem,