From cf69482d62d996d3ce840eeead8e160de281ac6c Mon Sep 17 00:00:00 2001 From: Waiman Long Date: Mon, 20 May 2019 16:59:11 -0400 Subject: locking/rwsem: Enable readers spinning on writer This patch enables readers to optimistically spin on a rwsem when it is owned by a writer instead of going to sleep directly. The rwsem_can_spin_on_owner() function is extracted out of rwsem_optimistic_spin() and is called directly by rwsem_down_read_slowpath() and rwsem_down_write_slowpath(). With a locking microbenchmark running on 5.1 based kernel, the total locking rates (in kops/s) on a 8-socket IvyBrige-EX system with equal numbers of readers and writers before and after the patch were as follows: # of Threads Pre-patch Post-patch ------------ --------- ---------- 4 1,674 1,684 8 1,062 1,074 16 924 900 32 300 458 64 195 208 128 164 168 240 149 143 The performance change wasn't significant in this case, but this change is required by a follow-on patch. Signed-off-by: Waiman Long Signed-off-by: Peter Zijlstra (Intel) Cc: Borislav Petkov Cc: Davidlohr Bueso Cc: H. Peter Anvin Cc: Linus Torvalds Cc: Peter Zijlstra Cc: Thomas Gleixner Cc: Tim Chen Cc: Will Deacon Cc: huang ying Link: https://lkml.kernel.org/r/20190520205918.22251-13-longman@redhat.com Signed-off-by: Ingo Molnar --- kernel/locking/lock_events_list.h | 1 + kernel/locking/rwsem.c | 86 +++++++++++++++++++++++++++++++++------ 2 files changed, 75 insertions(+), 12 deletions(-) (limited to 'kernel/locking') diff --git a/kernel/locking/lock_events_list.h b/kernel/locking/lock_events_list.h index 634b47fd8b5e..ca954e4e00e4 100644 --- a/kernel/locking/lock_events_list.h +++ b/kernel/locking/lock_events_list.h @@ -56,6 +56,7 @@ LOCK_EVENT(rwsem_sleep_reader) /* # of reader sleeps */ LOCK_EVENT(rwsem_sleep_writer) /* # of writer sleeps */ LOCK_EVENT(rwsem_wake_reader) /* # of reader wakeups */ LOCK_EVENT(rwsem_wake_writer) /* # of writer wakeups */ +LOCK_EVENT(rwsem_opt_rlock) /* # of read locks opt-spin acquired */ LOCK_EVENT(rwsem_opt_wlock) /* # of write locks opt-spin acquired */ LOCK_EVENT(rwsem_opt_fail) /* # of failed opt-spinnings */ LOCK_EVENT(rwsem_rlock) /* # of read locks acquired */ diff --git a/kernel/locking/rwsem.c b/kernel/locking/rwsem.c index 180455b6b0d4..985a03ad3f8c 100644 --- a/kernel/locking/rwsem.c +++ b/kernel/locking/rwsem.c @@ -457,6 +457,30 @@ static inline bool rwsem_try_write_lock(struct rw_semaphore *sem, } #ifdef CONFIG_RWSEM_SPIN_ON_OWNER +/* + * Try to acquire read lock before the reader is put on wait queue. + * Lock acquisition isn't allowed if the rwsem is locked or a writer handoff + * is ongoing. + */ +static inline bool rwsem_try_read_lock_unqueued(struct rw_semaphore *sem) +{ + long count = atomic_long_read(&sem->count); + + if (count & (RWSEM_WRITER_MASK | RWSEM_FLAG_HANDOFF)) + return false; + + count = atomic_long_fetch_add_acquire(RWSEM_READER_BIAS, &sem->count); + if (!(count & (RWSEM_WRITER_MASK | RWSEM_FLAG_HANDOFF))) { + rwsem_set_reader_owned(sem); + lockevent_inc(rwsem_opt_rlock); + return true; + } + + /* Back out the change */ + atomic_long_add(-RWSEM_READER_BIAS, &sem->count); + return false; +} + /* * Try to acquire write lock before the writer has been put on wait queue. */ @@ -491,9 +515,12 @@ static inline bool rwsem_can_spin_on_owner(struct rw_semaphore *sem) BUILD_BUG_ON(is_rwsem_owner_spinnable(RWSEM_OWNER_UNKNOWN)); - if (need_resched()) + if (need_resched()) { + lockevent_inc(rwsem_opt_fail); return false; + } + preempt_disable(); rcu_read_lock(); owner = READ_ONCE(sem->owner); if (owner) { @@ -501,6 +528,9 @@ static inline bool rwsem_can_spin_on_owner(struct rw_semaphore *sem) owner_on_cpu(owner); } rcu_read_unlock(); + preempt_enable(); + + lockevent_cond_inc(rwsem_opt_fail, !ret); return ret; } @@ -578,7 +608,7 @@ static noinline enum owner_state rwsem_spin_on_owner(struct rw_semaphore *sem) return state; } -static bool rwsem_optimistic_spin(struct rw_semaphore *sem) +static bool rwsem_optimistic_spin(struct rw_semaphore *sem, bool wlock) { bool taken = false; int prev_owner_state = OWNER_NULL; @@ -586,9 +616,6 @@ static bool rwsem_optimistic_spin(struct rw_semaphore *sem) preempt_disable(); /* sem->wait_lock should not be held when doing optimistic spinning */ - if (!rwsem_can_spin_on_owner(sem)) - goto done; - if (!osq_lock(&sem->osq)) goto done; @@ -608,10 +635,11 @@ static bool rwsem_optimistic_spin(struct rw_semaphore *sem) /* * Try to acquire the lock */ - if (rwsem_try_write_lock_unqueued(sem)) { - taken = true; + taken = wlock ? rwsem_try_write_lock_unqueued(sem) + : rwsem_try_read_lock_unqueued(sem); + + if (taken) break; - } /* * An RT task cannot do optimistic spinning if it cannot @@ -668,7 +696,12 @@ done: return taken; } #else -static bool rwsem_optimistic_spin(struct rw_semaphore *sem) +static inline bool rwsem_can_spin_on_owner(struct rw_semaphore *sem) +{ + return false; +} + +static inline bool rwsem_optimistic_spin(struct rw_semaphore *sem, bool wlock) { return false; } @@ -684,6 +717,31 @@ rwsem_down_read_slowpath(struct rw_semaphore *sem, int state) struct rwsem_waiter waiter; DEFINE_WAKE_Q(wake_q); + if (!rwsem_can_spin_on_owner(sem)) + goto queue; + + /* + * Undo read bias from down_read() and do optimistic spinning. + */ + atomic_long_add(-RWSEM_READER_BIAS, &sem->count); + adjustment = 0; + if (rwsem_optimistic_spin(sem, false)) { + /* + * Wake up other readers in the wait list if the front + * waiter is a reader. + */ + if ((atomic_long_read(&sem->count) & RWSEM_FLAG_WAITERS)) { + raw_spin_lock_irq(&sem->wait_lock); + if (!list_empty(&sem->wait_list)) + rwsem_mark_wake(sem, RWSEM_WAKE_READ_OWNED, + &wake_q); + raw_spin_unlock_irq(&sem->wait_lock); + wake_up_q(&wake_q); + } + return sem; + } + +queue: waiter.task = current; waiter.type = RWSEM_WAITING_FOR_READ; waiter.timeout = jiffies + RWSEM_WAIT_TIMEOUT; @@ -696,7 +754,7 @@ rwsem_down_read_slowpath(struct rw_semaphore *sem, int state) * exit the slowpath and return immediately as its * RWSEM_READER_BIAS has already been set in the count. */ - if (!(atomic_long_read(&sem->count) & + if (adjustment && !(atomic_long_read(&sem->count) & (RWSEM_WRITER_MASK | RWSEM_FLAG_HANDOFF))) { raw_spin_unlock_irq(&sem->wait_lock); rwsem_set_reader_owned(sem); @@ -708,7 +766,10 @@ rwsem_down_read_slowpath(struct rw_semaphore *sem, int state) list_add_tail(&waiter.list, &sem->wait_list); /* we're now waiting on the lock, but no longer actively locking */ - count = atomic_long_add_return(adjustment, &sem->count); + if (adjustment) + count = atomic_long_add_return(adjustment, &sem->count); + else + count = atomic_long_read(&sem->count); /* * If there are no active locks, wake the front queued process(es). @@ -767,7 +828,8 @@ rwsem_down_write_slowpath(struct rw_semaphore *sem, int state) DEFINE_WAKE_Q(wake_q); /* do optimistic spinning and steal lock if possible */ - if (rwsem_optimistic_spin(sem)) + if (rwsem_can_spin_on_owner(sem) && + rwsem_optimistic_spin(sem, true)) return sem; /* -- cgit v1.2.3