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![Alex Shi](/assets/img/avatar_default.png)
Commit 5a505085f0
("mm/rmap: Convert the struct anon_vma::mutex
to an rwsem") changed struct anon_vma::mutex to an rwsem, which
caused aim7 fork_test performance to drop by 50%.
Yuanhan Liu did the following excellent analysis:
https://lkml.org/lkml/2013/1/29/84
and found that the regression is caused by strict, serialized,
FIFO sequential write-ownership of rwsems. Ingo suggested
implementing opportunistic lock-stealing for the front writer
task in the waitqueue.
Yuanhan Liu implemented lock-stealing for spinlock-rwsems,
which indeed recovered much of the regression - confirming
the analysis that the main factor in the regression was the
FIFO writer-fairness of rwsems.
In this patch we allow lock-stealing to happen when the first
waiter is also writer. With that change in place the
aim7 fork_test performance is fully recovered on my
Intel NHM EP, NHM EX, SNB EP 2S and 4S test-machines.
Reported-by: lkp@linux.intel.com
Reported-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>
Signed-off-by: Alex Shi <alex.shi@intel.com>
Cc: David Howells <dhowells@redhat.com>
Cc: Michel Lespinasse <walken@google.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Anton Blanchard <anton@samba.org>
Cc: Arjan van de Ven <arjan@linux.intel.com>
Cc: paul.gortmaker@windriver.com
Link: https://lkml.org/lkml/2013/1/29/84
Link: http://lkml.kernel.org/r/1360069915-31619-1-git-send-email-alex.shi@intel.com
[ Small stylistic fixes, updated changelog. ]
Signed-off-by: Ingo Molnar <mingo@kernel.org>
299 lines
8.5 KiB
C
299 lines
8.5 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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*/
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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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struct rwsem_waiter {
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struct list_head list;
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struct task_struct *task;
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unsigned int flags;
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#define RWSEM_WAITING_FOR_READ 0x00000001
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#define RWSEM_WAITING_FOR_WRITE 0x00000002
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};
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/* Wake types for __rwsem_do_wake(). Note that RWSEM_WAKE_NO_ACTIVE and
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* RWSEM_WAKE_READ_OWNED imply that the spinlock must have been kept held
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* since the rwsem value was observed.
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*/
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#define RWSEM_WAKE_ANY 0 /* Wake whatever's at head of wait list */
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#define RWSEM_WAKE_NO_ACTIVE 1 /* rwsem was observed with no active thread */
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#define RWSEM_WAKE_READ_OWNED 2 /* rwsem was observed to be read owned */
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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, int 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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signed long woken, loop, adjustment;
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waiter = list_entry(sem->wait_list.next, struct rwsem_waiter, list);
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if (!(waiter->flags & RWSEM_WAITING_FOR_WRITE))
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goto readers_only;
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if (wake_type == RWSEM_WAKE_READ_OWNED)
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/* Another active reader was observed, so wakeup is not
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* likely to succeed. Save the atomic op.
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*/
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goto out;
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/* Wake up the writing waiter and let the task grab the sem: */
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wake_up_process(waiter->task);
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goto out;
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readers_only:
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/* If we come here from up_xxxx(), another thread might have reached
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* rwsem_down_failed_common() before we acquired the spinlock and
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* woken up a waiter, making it now active. We prefer to check for
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* this first in order to not spend too much time with the spinlock
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* held if we're not going to be able to wake up readers in the end.
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*
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* Note that we do not need to update the rwsem count: any writer
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* trying to acquire rwsem will run rwsem_down_write_failed() due
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* to the waiting threads and block trying to acquire the spinlock.
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*
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* We use a dummy atomic update in order to acquire the cache line
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* exclusively since we expect to succeed and run the final rwsem
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* count adjustment pretty soon.
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*/
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if (wake_type == RWSEM_WAKE_ANY &&
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rwsem_atomic_update(0, sem) < RWSEM_WAITING_BIAS)
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/* Someone grabbed the sem for write already */
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goto out;
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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->flags & RWSEM_WAITING_FOR_READ);
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adjustment = woken * RWSEM_ACTIVE_READ_BIAS;
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if (waiter->flags & RWSEM_WAITING_FOR_READ)
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/* hit end of list above */
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adjustment -= RWSEM_WAITING_BIAS;
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rwsem_atomic_add(adjustment, sem);
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next = sem->wait_list.next;
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for (loop = woken; loop > 0; loop--) {
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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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}
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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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/* Try to get write sem, caller holds sem->wait_lock: */
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static int try_get_writer_sem(struct rw_semaphore *sem,
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struct rwsem_waiter *waiter)
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{
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struct rwsem_waiter *fwaiter;
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long oldcount, adjustment;
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/* only steal when first waiter is writing */
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fwaiter = list_entry(sem->wait_list.next, struct rwsem_waiter, list);
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if (!(fwaiter->flags & RWSEM_WAITING_FOR_WRITE))
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return 0;
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adjustment = RWSEM_ACTIVE_WRITE_BIAS;
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/* Only one waiter in the queue: */
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if (fwaiter == waiter && waiter->list.next == &sem->wait_list)
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adjustment -= RWSEM_WAITING_BIAS;
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try_again_write:
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oldcount = rwsem_atomic_update(adjustment, sem) - adjustment;
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if (!(oldcount & RWSEM_ACTIVE_MASK)) {
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/* No active lock: */
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struct task_struct *tsk = waiter->task;
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list_del(&waiter->list);
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smp_mb();
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put_task_struct(tsk);
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tsk->state = TASK_RUNNING;
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return 1;
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}
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/* some one grabbed the sem already */
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if (rwsem_atomic_update(-adjustment, sem) & RWSEM_ACTIVE_MASK)
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return 0;
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goto try_again_write;
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}
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/*
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* wait for a lock to be granted
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*/
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static struct rw_semaphore __sched *
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rwsem_down_failed_common(struct rw_semaphore *sem,
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unsigned int flags, signed long adjustment)
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{
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struct rwsem_waiter waiter;
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struct task_struct *tsk = current;
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signed long count;
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set_task_state(tsk, TASK_UNINTERRUPTIBLE);
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/* set up my own style of waitqueue */
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raw_spin_lock_irq(&sem->wait_lock);
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waiter.task = tsk;
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waiter.flags = flags;
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get_task_struct(tsk);
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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) up.
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*
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* Alternatively, if we're called from a failed down_write(), there
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* were already threads queued before us and there are no active
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* writers, the lock must be read owned; so we try to wake any read
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* locks that were queued ahead of us. */
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if (count == RWSEM_WAITING_BIAS)
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sem = __rwsem_do_wake(sem, RWSEM_WAKE_NO_ACTIVE);
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else 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_READ_OWNED);
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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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for (;;) {
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if (!waiter.task)
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break;
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raw_spin_lock_irq(&sem->wait_lock);
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/* Try to get the writer sem, may steal from the head writer: */
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if (flags == RWSEM_WAITING_FOR_WRITE)
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if (try_get_writer_sem(sem, &waiter)) {
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raw_spin_unlock_irq(&sem->wait_lock);
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return sem;
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}
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raw_spin_unlock_irq(&sem->wait_lock);
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schedule();
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set_task_state(tsk, TASK_UNINTERRUPTIBLE);
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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 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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return rwsem_down_failed_common(sem, RWSEM_WAITING_FOR_READ,
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-RWSEM_ACTIVE_READ_BIAS);
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}
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/*
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* wait for the write lock to be granted
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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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return rwsem_down_failed_common(sem, RWSEM_WAITING_FOR_WRITE,
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-RWSEM_ACTIVE_WRITE_BIAS);
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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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