From 80127a39681bd68c959f0953f84a830cbd7c3b1c Mon Sep 17 00:00:00 2001 From: Peter Zijlstra Date: Thu, 14 Jul 2016 20:08:46 +0200 Subject: 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) Reviewed-by: Oleg Nesterov Cc: Andrew Morton Cc: Linus Torvalds Cc: Paul E. McKenney Cc: Paul McKenney Cc: Peter Zijlstra Cc: Thomas Gleixner Cc: linux-kernel@vger.kernel.org [ Fixed modular build. ] Signed-off-by: Ingo Molnar --- kernel/locking/percpu-rwsem.c | 228 ++++++++++++++++++++++++------------------ 1 file changed, 131 insertions(+), 97 deletions(-) (limited to 'kernel/locking/percpu-rwsem.c') diff --git a/kernel/locking/percpu-rwsem.c b/kernel/locking/percpu-rwsem.c index bec0b647f9cc..ce182599cf2e 100644 --- a/kernel/locking/percpu-rwsem.c +++ b/kernel/locking/percpu-rwsem.c @@ -8,152 +8,186 @@ #include #include -int __percpu_init_rwsem(struct percpu_rw_semaphore *brw, +int __percpu_init_rwsem(struct percpu_rw_semaphore *sem, const char *name, struct lock_class_key *rwsem_key) { - brw->fast_read_ctr = alloc_percpu(int); - if (unlikely(!brw->fast_read_ctr)) + sem->read_count = alloc_percpu(int); + if (unlikely(!sem->read_count)) return -ENOMEM; /* ->rw_sem represents the whole percpu_rw_semaphore for lockdep */ - __init_rwsem(&brw->rw_sem, name, rwsem_key); - rcu_sync_init(&brw->rss, RCU_SCHED_SYNC); - atomic_set(&brw->slow_read_ctr, 0); - init_waitqueue_head(&brw->write_waitq); + rcu_sync_init(&sem->rss, RCU_SCHED_SYNC); + __init_rwsem(&sem->rw_sem, name, rwsem_key); + init_waitqueue_head(&sem->writer); + sem->readers_block = 0; return 0; } EXPORT_SYMBOL_GPL(__percpu_init_rwsem); -void percpu_free_rwsem(struct percpu_rw_semaphore *brw) +void percpu_free_rwsem(struct percpu_rw_semaphore *sem) { /* * XXX: temporary kludge. The error path in alloc_super() * assumes that percpu_free_rwsem() is safe after kzalloc(). */ - if (!brw->fast_read_ctr) + if (!sem->read_count) return; - rcu_sync_dtor(&brw->rss); - free_percpu(brw->fast_read_ctr); - brw->fast_read_ctr = NULL; /* catch use after free bugs */ + rcu_sync_dtor(&sem->rss); + free_percpu(sem->read_count); + sem->read_count = NULL; /* catch use after free bugs */ } EXPORT_SYMBOL_GPL(percpu_free_rwsem); -/* - * This is the fast-path for down_read/up_read. If it succeeds we rely - * on the barriers provided by rcu_sync_enter/exit; see the comments in - * percpu_down_write() and percpu_up_write(). - * - * If this helper fails the callers rely on the normal rw_semaphore and - * atomic_dec_and_test(), so in this case we have the necessary barriers. - */ -static bool update_fast_ctr(struct percpu_rw_semaphore *brw, unsigned int val) +int __percpu_down_read(struct percpu_rw_semaphore *sem, int try) { - bool success; + /* + * Due to having preemption disabled the decrement happens on + * the same CPU as the increment, avoiding the + * increment-on-one-CPU-and-decrement-on-another problem. + * + * If the reader misses the writer's assignment of readers_block, then + * the writer is guaranteed to see the reader's increment. + * + * Conversely, any readers that increment their sem->read_count after + * the writer looks are guaranteed to see the readers_block value, + * which in turn means that they are guaranteed to immediately + * decrement their sem->read_count, so that it doesn't matter that the + * writer missed them. + */ - preempt_disable(); - success = rcu_sync_is_idle(&brw->rss); - if (likely(success)) - __this_cpu_add(*brw->fast_read_ctr, val); - preempt_enable(); + smp_mb(); /* A matches D */ - return success; -} + /* + * If !readers_block the critical section starts here, matched by the + * release in percpu_up_write(). + */ + if (likely(!smp_load_acquire(&sem->readers_block))) + return 1; -/* - * Like the normal down_read() this is not recursive, the writer can - * come after the first percpu_down_read() and create the deadlock. - * - * Note: returns with lock_is_held(brw->rw_sem) == T for lockdep, - * percpu_up_read() does rwsem_release(). This pairs with the usage - * of ->rw_sem in percpu_down/up_write(). - */ -void percpu_down_read(struct percpu_rw_semaphore *brw) -{ - might_sleep(); - rwsem_acquire_read(&brw->rw_sem.dep_map, 0, 0, _RET_IP_); + /* + * Per the above comment; we still have preemption disabled and + * will thus decrement on the same CPU as we incremented. + */ + __percpu_up_read(sem); - if (likely(update_fast_ctr(brw, +1))) - return; + if (try) + return 0; - /* Avoid rwsem_acquire_read() and rwsem_release() */ - __down_read(&brw->rw_sem); - atomic_inc(&brw->slow_read_ctr); - __up_read(&brw->rw_sem); -} -EXPORT_SYMBOL_GPL(percpu_down_read); + /* + * We either call schedule() in the wait, or we'll fall through + * and reschedule on the preempt_enable() in percpu_down_read(). + */ + preempt_enable_no_resched(); -int percpu_down_read_trylock(struct percpu_rw_semaphore *brw) -{ - if (unlikely(!update_fast_ctr(brw, +1))) { - if (!__down_read_trylock(&brw->rw_sem)) - return 0; - atomic_inc(&brw->slow_read_ctr); - __up_read(&brw->rw_sem); - } - - rwsem_acquire_read(&brw->rw_sem.dep_map, 0, 1, _RET_IP_); + /* + * Avoid lockdep for the down/up_read() we already have them. + */ + __down_read(&sem->rw_sem); + this_cpu_inc(*sem->read_count); + __up_read(&sem->rw_sem); + + preempt_disable(); return 1; } +EXPORT_SYMBOL_GPL(__percpu_down_read); -void percpu_up_read(struct percpu_rw_semaphore *brw) +void __percpu_up_read(struct percpu_rw_semaphore *sem) { - rwsem_release(&brw->rw_sem.dep_map, 1, _RET_IP_); - - if (likely(update_fast_ctr(brw, -1))) - return; + smp_mb(); /* B matches C */ + /* + * In other words, if they see our decrement (presumably to aggregate + * zero, as that is the only time it matters) they will also see our + * critical section. + */ + __this_cpu_dec(*sem->read_count); - /* false-positive is possible but harmless */ - if (atomic_dec_and_test(&brw->slow_read_ctr)) - wake_up_all(&brw->write_waitq); + /* Prod writer to recheck readers_active */ + wake_up(&sem->writer); } -EXPORT_SYMBOL_GPL(percpu_up_read); +EXPORT_SYMBOL_GPL(__percpu_up_read); + +#define per_cpu_sum(var) \ +({ \ + typeof(var) __sum = 0; \ + int cpu; \ + compiletime_assert_atomic_type(__sum); \ + for_each_possible_cpu(cpu) \ + __sum += per_cpu(var, cpu); \ + __sum; \ +}) -static int clear_fast_ctr(struct percpu_rw_semaphore *brw) +/* + * Return true if the modular sum of the sem->read_count per-CPU variable is + * zero. If this sum is zero, then it is stable due to the fact that if any + * newly arriving readers increment a given counter, they will immediately + * decrement that same counter. + */ +static bool readers_active_check(struct percpu_rw_semaphore *sem) { - unsigned int sum = 0; - int cpu; + if (per_cpu_sum(*sem->read_count) != 0) + return false; + + /* + * If we observed the decrement; ensure we see the entire critical + * section. + */ - for_each_possible_cpu(cpu) { - sum += per_cpu(*brw->fast_read_ctr, cpu); - per_cpu(*brw->fast_read_ctr, cpu) = 0; - } + smp_mb(); /* C matches B */ - return sum; + return true; } -void percpu_down_write(struct percpu_rw_semaphore *brw) +void percpu_down_write(struct percpu_rw_semaphore *sem) { + /* Notify readers to take the slow path. */ + rcu_sync_enter(&sem->rss); + + down_write(&sem->rw_sem); + /* - * Make rcu_sync_is_idle() == F and thus disable the fast-path in - * percpu_down_read() and percpu_up_read(), and wait for gp pass. - * - * The latter synchronises us with the preceding readers which used - * the fast-past, so we can not miss the result of __this_cpu_add() - * or anything else inside their criticial sections. + * Notify new readers to block; up until now, and thus throughout the + * longish rcu_sync_enter() above, new readers could still come in. */ - rcu_sync_enter(&brw->rss); + WRITE_ONCE(sem->readers_block, 1); - /* exclude other writers, and block the new readers completely */ - down_write(&brw->rw_sem); + smp_mb(); /* D matches A */ - /* nobody can use fast_read_ctr, move its sum into slow_read_ctr */ - atomic_add(clear_fast_ctr(brw), &brw->slow_read_ctr); + /* + * If they don't see our writer of readers_block, then we are + * guaranteed to see their sem->read_count increment, and therefore + * will wait for them. + */ - /* wait for all readers to complete their percpu_up_read() */ - wait_event(brw->write_waitq, !atomic_read(&brw->slow_read_ctr)); + /* Wait for all now active readers to complete. */ + wait_event(sem->writer, readers_active_check(sem)); } EXPORT_SYMBOL_GPL(percpu_down_write); -void percpu_up_write(struct percpu_rw_semaphore *brw) +void percpu_up_write(struct percpu_rw_semaphore *sem) { - /* release the lock, but the readers can't use the fast-path */ - up_write(&brw->rw_sem); /* - * Enable the fast-path in percpu_down_read() and percpu_up_read() - * but only after another gp pass; this adds the necessary barrier - * to ensure the reader can't miss the changes done by us. + * Signal the writer is done, no fast path yet. + * + * One reason that we cannot just immediately flip to readers_fast is + * that new readers might fail to see the results of this writer's + * critical section. + * + * Therefore we force it through the slow path which guarantees an + * acquire and thereby guarantees the critical section's consistency. + */ + smp_store_release(&sem->readers_block, 0); + + /* + * Release the write lock, this will allow readers back in the game. + */ + up_write(&sem->rw_sem); + + /* + * Once this completes (at least one RCU-sched grace period hence) the + * reader fast path will be available again. Safe to use outside the + * exclusive write lock because its counting. */ - rcu_sync_exit(&brw->rss); + rcu_sync_exit(&sem->rss); } EXPORT_SYMBOL_GPL(percpu_up_write); -- cgit v1.2.3-70-g09d2