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![Davidlohr Bueso](/assets/img/avatar_default.png)
This patch tries to reduce the amount of cmpxchg calls in the writer failed path by checking the counter value first before issuing the instruction. If ->count is not set to RWSEM_WAITING_BIAS then there is no point wasting a cmpxchg call. Furthermore, Michel states "I suppose it helps due to the case where someone else steals the lock while we're trying to acquire sem->wait_lock." Two very different workloads and machines were used to see how this patch improves throughput: pgbench on a quad-core laptop and aim7 on a large 8 socket box with 80 cores. Some results comparing Michel's fast-path write lock stealing (tps-rwsem) on a quad-core laptop running pgbench: | db_size | clients | tps-rwsem | tps-patch | +---------+----------+----------------+--------------+ | 160 MB | 1 | 6906 | 9153 | + 32.5 | 160 MB | 2 | 15931 | 22487 | + 41.1% | 160 MB | 4 | 33021 | 32503 | | 160 MB | 8 | 34626 | 34695 | | 160 MB | 16 | 33098 | 34003 | | 160 MB | 20 | 31343 | 31440 | | 160 MB | 30 | 28961 | 28987 | | 160 MB | 40 | 26902 | 26970 | | 160 MB | 50 | 25760 | 25810 | ------------------------------------------------------ | 1.6 GB | 1 | 7729 | 7537 | | 1.6 GB | 2 | 19009 | 23508 | + 23.7% | 1.6 GB | 4 | 33185 | 32666 | | 1.6 GB | 8 | 34550 | 34318 | | 1.6 GB | 16 | 33079 | 32689 | | 1.6 GB | 20 | 31494 | 31702 | | 1.6 GB | 30 | 28535 | 28755 | | 1.6 GB | 40 | 27054 | 27017 | | 1.6 GB | 50 | 25591 | 25560 | ------------------------------------------------------ | 7.6 GB | 1 | 6224 | 7469 | + 20.0% | 7.6 GB | 2 | 13611 | 12778 | | 7.6 GB | 4 | 33108 | 32927 | | 7.6 GB | 8 | 34712 | 34878 | | 7.6 GB | 16 | 32895 | 33003 | | 7.6 GB | 20 | 31689 | 31974 | | 7.6 GB | 30 | 29003 | 28806 | | 7.6 GB | 40 | 26683 | 26976 | | 7.6 GB | 50 | 25925 | 25652 | ------------------------------------------------------ For the aim7 worloads, they overall improved on top of Michel's patchset. For full graphs on how the rwsem series plus this patch behaves on a large 8 socket machine against a vanilla kernel: http://stgolabs.net/rwsem-aim7-results.tar.gz Signed-off-by: Davidlohr Bueso <davidlohr.bueso@hp.com> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
293 lines
7.9 KiB
C
293 lines
7.9 KiB
C
/* rwsem.c: R/W semaphores: contention handling functions
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*
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* Written by David Howells (dhowells@redhat.com).
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* Derived from arch/i386/kernel/semaphore.c
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*
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* Writer lock-stealing by Alex Shi <alex.shi@intel.com>
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* and Michel Lespinasse <walken@google.com>
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*/
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#include <linux/rwsem.h>
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#include <linux/sched.h>
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#include <linux/init.h>
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#include <linux/export.h>
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/*
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* Initialize an rwsem:
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*/
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void __init_rwsem(struct rw_semaphore *sem, const char *name,
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struct lock_class_key *key)
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{
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#ifdef CONFIG_DEBUG_LOCK_ALLOC
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/*
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* Make sure we are not reinitializing a held semaphore:
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*/
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debug_check_no_locks_freed((void *)sem, sizeof(*sem));
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lockdep_init_map(&sem->dep_map, name, key, 0);
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#endif
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sem->count = RWSEM_UNLOCKED_VALUE;
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raw_spin_lock_init(&sem->wait_lock);
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INIT_LIST_HEAD(&sem->wait_list);
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}
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EXPORT_SYMBOL(__init_rwsem);
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enum rwsem_waiter_type {
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RWSEM_WAITING_FOR_WRITE,
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RWSEM_WAITING_FOR_READ
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};
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struct rwsem_waiter {
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struct list_head list;
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struct task_struct *task;
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enum rwsem_waiter_type type;
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};
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enum rwsem_wake_type {
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RWSEM_WAKE_ANY, /* Wake whatever's at head of wait list */
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RWSEM_WAKE_READERS, /* Wake readers only */
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RWSEM_WAKE_READ_OWNED /* Waker thread holds the read lock */
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};
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/*
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* handle the lock release when processes blocked on it that can now run
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* - if we come here from up_xxxx(), then:
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* - the 'active part' of count (&0x0000ffff) reached 0 (but may have changed)
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* - the 'waiting part' of count (&0xffff0000) is -ve (and will still be so)
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* - there must be someone on the queue
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* - the spinlock must be held by the caller
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* - woken process blocks are discarded from the list after having task zeroed
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* - writers are only woken if downgrading is false
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*/
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static struct rw_semaphore *
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__rwsem_do_wake(struct rw_semaphore *sem, enum rwsem_wake_type wake_type)
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{
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struct rwsem_waiter *waiter;
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struct task_struct *tsk;
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struct list_head *next;
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long oldcount, woken, loop, adjustment;
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waiter = list_entry(sem->wait_list.next, struct rwsem_waiter, list);
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if (waiter->type == RWSEM_WAITING_FOR_WRITE) {
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if (wake_type == RWSEM_WAKE_ANY)
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/* Wake writer at the front of the queue, but do not
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* grant it the lock yet as we want other writers
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* to be able to steal it. Readers, on the other hand,
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* will block as they will notice the queued writer.
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*/
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wake_up_process(waiter->task);
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goto out;
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}
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/* Writers might steal the lock before we grant it to the next reader.
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* We prefer to do the first reader grant before counting readers
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* so we can bail out early if a writer stole the lock.
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*/
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adjustment = 0;
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if (wake_type != RWSEM_WAKE_READ_OWNED) {
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adjustment = RWSEM_ACTIVE_READ_BIAS;
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try_reader_grant:
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oldcount = rwsem_atomic_update(adjustment, sem) - adjustment;
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if (unlikely(oldcount < RWSEM_WAITING_BIAS)) {
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/* A writer stole the lock. Undo our reader grant. */
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if (rwsem_atomic_update(-adjustment, sem) &
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RWSEM_ACTIVE_MASK)
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goto out;
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/* Last active locker left. Retry waking readers. */
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goto try_reader_grant;
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}
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}
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/* Grant an infinite number of read locks to the readers at the front
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* of the queue. Note we increment the 'active part' of the count by
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* the number of readers before waking any processes up.
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*/
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woken = 0;
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do {
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woken++;
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if (waiter->list.next == &sem->wait_list)
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break;
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waiter = list_entry(waiter->list.next,
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struct rwsem_waiter, list);
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} while (waiter->type != RWSEM_WAITING_FOR_WRITE);
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adjustment = woken * RWSEM_ACTIVE_READ_BIAS - adjustment;
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if (waiter->type != RWSEM_WAITING_FOR_WRITE)
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/* hit end of list above */
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adjustment -= RWSEM_WAITING_BIAS;
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if (adjustment)
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rwsem_atomic_add(adjustment, sem);
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next = sem->wait_list.next;
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loop = woken;
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do {
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waiter = list_entry(next, struct rwsem_waiter, list);
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next = waiter->list.next;
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tsk = waiter->task;
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smp_mb();
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waiter->task = NULL;
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wake_up_process(tsk);
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put_task_struct(tsk);
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} while (--loop);
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sem->wait_list.next = next;
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next->prev = &sem->wait_list;
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out:
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return sem;
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}
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/*
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* wait for the read lock to be granted
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*/
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struct rw_semaphore __sched *rwsem_down_read_failed(struct rw_semaphore *sem)
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{
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long count, adjustment = -RWSEM_ACTIVE_READ_BIAS;
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struct rwsem_waiter waiter;
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struct task_struct *tsk = current;
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/* set up my own style of waitqueue */
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waiter.task = tsk;
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waiter.type = RWSEM_WAITING_FOR_READ;
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get_task_struct(tsk);
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raw_spin_lock_irq(&sem->wait_lock);
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if (list_empty(&sem->wait_list))
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adjustment += RWSEM_WAITING_BIAS;
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list_add_tail(&waiter.list, &sem->wait_list);
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/* we're now waiting on the lock, but no longer actively locking */
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count = rwsem_atomic_update(adjustment, sem);
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/* If there are no active locks, wake the front queued process(es).
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*
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* If there are no writers and we are first in the queue,
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* wake our own waiter to join the existing active readers !
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*/
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if (count == RWSEM_WAITING_BIAS ||
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(count > RWSEM_WAITING_BIAS &&
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adjustment != -RWSEM_ACTIVE_READ_BIAS))
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sem = __rwsem_do_wake(sem, RWSEM_WAKE_ANY);
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raw_spin_unlock_irq(&sem->wait_lock);
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/* wait to be given the lock */
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while (true) {
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set_task_state(tsk, TASK_UNINTERRUPTIBLE);
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if (!waiter.task)
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break;
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schedule();
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}
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tsk->state = TASK_RUNNING;
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return sem;
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}
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/*
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* wait until we successfully acquire the write lock
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*/
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struct rw_semaphore __sched *rwsem_down_write_failed(struct rw_semaphore *sem)
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{
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long count, adjustment = -RWSEM_ACTIVE_WRITE_BIAS;
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struct rwsem_waiter waiter;
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struct task_struct *tsk = current;
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/* set up my own style of waitqueue */
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waiter.task = tsk;
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waiter.type = RWSEM_WAITING_FOR_WRITE;
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raw_spin_lock_irq(&sem->wait_lock);
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if (list_empty(&sem->wait_list))
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adjustment += RWSEM_WAITING_BIAS;
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list_add_tail(&waiter.list, &sem->wait_list);
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/* we're now waiting on the lock, but no longer actively locking */
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count = rwsem_atomic_update(adjustment, sem);
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/* If there were already threads queued before us and there are no
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* active writers, the lock must be read owned; so we try to wake
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* any read locks that were queued ahead of us. */
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if (count > RWSEM_WAITING_BIAS &&
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adjustment == -RWSEM_ACTIVE_WRITE_BIAS)
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sem = __rwsem_do_wake(sem, RWSEM_WAKE_READERS);
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/* wait until we successfully acquire the lock */
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set_task_state(tsk, TASK_UNINTERRUPTIBLE);
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while (true) {
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if (!(count & RWSEM_ACTIVE_MASK)) {
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/* Try acquiring the write lock. */
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count = RWSEM_ACTIVE_WRITE_BIAS;
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if (!list_is_singular(&sem->wait_list))
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count += RWSEM_WAITING_BIAS;
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if (sem->count == RWSEM_WAITING_BIAS &&
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cmpxchg(&sem->count, RWSEM_WAITING_BIAS, count) ==
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RWSEM_WAITING_BIAS)
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break;
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}
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raw_spin_unlock_irq(&sem->wait_lock);
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/* Block until there are no active lockers. */
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do {
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schedule();
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set_task_state(tsk, TASK_UNINTERRUPTIBLE);
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} while ((count = sem->count) & RWSEM_ACTIVE_MASK);
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raw_spin_lock_irq(&sem->wait_lock);
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}
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list_del(&waiter.list);
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raw_spin_unlock_irq(&sem->wait_lock);
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tsk->state = TASK_RUNNING;
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return sem;
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}
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/*
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* handle waking up a waiter on the semaphore
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* - up_read/up_write has decremented the active part of count if we come here
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*/
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struct rw_semaphore *rwsem_wake(struct rw_semaphore *sem)
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{
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unsigned long flags;
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raw_spin_lock_irqsave(&sem->wait_lock, flags);
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/* do nothing if list empty */
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if (!list_empty(&sem->wait_list))
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sem = __rwsem_do_wake(sem, RWSEM_WAKE_ANY);
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raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
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return sem;
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}
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/*
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* downgrade a write lock into a read lock
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* - caller incremented waiting part of count and discovered it still negative
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* - just wake up any readers at the front of the queue
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*/
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struct rw_semaphore *rwsem_downgrade_wake(struct rw_semaphore *sem)
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{
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unsigned long flags;
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raw_spin_lock_irqsave(&sem->wait_lock, flags);
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/* do nothing if list empty */
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if (!list_empty(&sem->wait_list))
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sem = __rwsem_do_wake(sem, RWSEM_WAKE_READ_OWNED);
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raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
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return sem;
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}
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EXPORT_SYMBOL(rwsem_down_read_failed);
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EXPORT_SYMBOL(rwsem_down_write_failed);
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EXPORT_SYMBOL(rwsem_wake);
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EXPORT_SYMBOL(rwsem_downgrade_wake);
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