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SubjectRe: Performance regression from switching lock to rw-sem for anon-vma tree
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cc'ing Dave Chinner for XFS

On Mon, 2013-07-29 at 17:13 -0700, Tim Chen wrote:
> On Tue, 2013-07-23 at 11:53 +0200, Ingo Molnar wrote:
> > * Peter Zijlstra <peterz@infradead.org> wrote:
> >
> > > On Tue, Jul 23, 2013 at 11:45:13AM +0200, Ingo Molnar wrote:
> > >
> > > > Why not just try the delayed addition approach first? The spinning is
> > > > time limited AFAICS, so we don't _have to_ recognize those as writers
> > > > per se, only if the spinning fails and it wants to go on the waitlist.
> > > > Am I missing something?
> > > >
> > > > It will change patterns, it might even change the fairness balance -
> > > > but is a legit change otherwise, especially if it helps performance.
> > >
> > > Be very careful here. Some people (XFS) have very specific needs. Walken
> > > and dchinner had a longish discussion on this a while back.
> >
> > Agreed - yet it's worth at least trying it out the quick way, to see the
> > main effect and to see whether that explains the performance assymetry and
> > invest more effort into it.
> >
>
> Ingo & Peter,
>
> Here's a patch that moved optimistic spinning of the writer lock
> acquisition before putting the writer on the wait list. It did give me
> a 5% performance boost on my exim mail server workload.
> It recovered a good portion of the 8% performance regression from
> mutex implementation.
>
> I think we are on the right track. Let me know if there's anything
> in the patch that may cause grief to XFS.
>
> There is some further optimization possible. We went
> to the failed path within __down_write if the count field is non
> zero. But in fact if the old count field was RWSEM_WAITING_BIAS,
> there's no one active and we could have stolen the
> lock when we perform our atomic op on the count field
> in __down_write. Yet we go to the failed path in the current
> code.
>
> I will combine this change and also Alex's patches on rwsem together
> in a patchset later.
>
> Your comments and thoughts are most welcomed.
>
> Tim
>
> Signed-off-by: Tim Chen <tim.c.chen@linux.intel.com>
> ---
> diff --git a/include/linux/rwsem.h b/include/linux/rwsem.h
> index 0616ffe..58a4acb 100644
> --- a/include/linux/rwsem.h
> +++ b/include/linux/rwsem.h
> @@ -29,6 +29,10 @@ struct rw_semaphore {
> #ifdef CONFIG_DEBUG_LOCK_ALLOC
> struct lockdep_map dep_map;
> #endif
> +#ifdef CONFIG_RWSEM_SPIN_ON_WRITE_OWNER
> + struct task_struct *owner;
> + void *spin_mlock;
> +#endif
> };
>
> extern struct rw_semaphore *rwsem_down_read_failed(struct rw_semaphore *sem);
> @@ -55,11 +59,21 @@ static inline int rwsem_is_locked(struct rw_semaphore *sem)
> # define __RWSEM_DEP_MAP_INIT(lockname)
> #endif
>
> +#ifdef CONFIG_RWSEM_SPIN_ON_WRITE_OWNER
> +#define __RWSEM_INITIALIZER(name) \
> + { RWSEM_UNLOCKED_VALUE, \
> + __RAW_SPIN_LOCK_UNLOCKED(name.wait_lock), \
> + LIST_HEAD_INIT((name).wait_list), \
> + __RWSEM_DEP_MAP_INIT(name) \
> + NULL, \
> + NULL }
> +#else
> #define __RWSEM_INITIALIZER(name) \
> { RWSEM_UNLOCKED_VALUE, \
> __RAW_SPIN_LOCK_UNLOCKED(name.wait_lock), \
> LIST_HEAD_INIT((name).wait_list) \
> __RWSEM_DEP_MAP_INIT(name) }
> +#endif
>
> #define DECLARE_RWSEM(name) \
> struct rw_semaphore name = __RWSEM_INITIALIZER(name)
> diff --git a/init/Kconfig b/init/Kconfig
> index 9d3a788..d97225f 100644
> --- a/init/Kconfig
> +++ b/init/Kconfig
> @@ -1595,6 +1595,16 @@ config TRACEPOINTS
>
> source "arch/Kconfig"
>
> +config RWSEM_SPIN_ON_WRITE_OWNER
> + bool "Optimistic spin write acquisition for writer owned rw-sem"
> + default n
> + depends on SMP
> + help
> + Allows a writer to perform optimistic spinning if another writer own
> + the read write semaphore. If the lock owner is running, it is likely
> + to release the lock soon. Spinning gives a greater chance for writer to
> + acquire a semaphore before putting it to sleep.
> +
> endmenu # General setup
>
> config HAVE_GENERIC_DMA_COHERENT
> diff --git a/kernel/rwsem.c b/kernel/rwsem.c
> index cfff143..a32990a 100644
> --- a/kernel/rwsem.c
> +++ b/kernel/rwsem.c
> @@ -12,6 +12,26 @@
>
> #include <linux/atomic.h>
>
> +#ifdef CONFIG_RWSEM_SPIN_ON_WRITE_OWNER
> +static inline void rwsem_set_owner(struct rw_semaphore *sem)
> +{
> + sem->owner = current;
> +}
> +
> +static inline void rwsem_clear_owner(struct rw_semaphore *sem)
> +{
> + sem->owner = NULL;
> +}
> +#else
> +static inline void rwsem_set_owner(struct rw_semaphore *sem)
> +{
> +}
> +
> +static inline void rwsem_clear_owner(struct rw_semaphore *sem)
> +{
> +}
> +#endif
> +
> /*
> * lock for reading
> */
> @@ -48,6 +68,7 @@ void __sched down_write(struct rw_semaphore *sem)
> rwsem_acquire(&sem->dep_map, 0, 0, _RET_IP_);
>
> LOCK_CONTENDED(sem, __down_write_trylock, __down_write);
> + rwsem_set_owner(sem);
> }
>
> EXPORT_SYMBOL(down_write);
> @@ -59,8 +80,10 @@ int down_write_trylock(struct rw_semaphore *sem)
> {
> int ret = __down_write_trylock(sem);
>
> - if (ret == 1)
> + if (ret == 1) {
> rwsem_acquire(&sem->dep_map, 0, 1, _RET_IP_);
> + rwsem_set_owner(sem);
> + }
> return ret;
> }
>
> @@ -86,6 +109,7 @@ void up_write(struct rw_semaphore *sem)
> rwsem_release(&sem->dep_map, 1, _RET_IP_);
>
> __up_write(sem);
> + rwsem_clear_owner(sem);
> }
>
> EXPORT_SYMBOL(up_write);
> @@ -100,6 +124,7 @@ void downgrade_write(struct rw_semaphore *sem)
> * dependency.
> */
> __downgrade_write(sem);
> + rwsem_clear_owner(sem);
> }
>
> EXPORT_SYMBOL(downgrade_write);
> @@ -122,6 +147,7 @@ void _down_write_nest_lock(struct rw_semaphore *sem, struct lockdep_map *nest)
> rwsem_acquire_nest(&sem->dep_map, 0, 0, nest, _RET_IP_);
>
> LOCK_CONTENDED(sem, __down_write_trylock, __down_write);
> + rwsem_set_owner(sem);
> }
>
> EXPORT_SYMBOL(_down_write_nest_lock);
> @@ -141,6 +167,7 @@ void down_write_nested(struct rw_semaphore *sem, int subclass)
> rwsem_acquire(&sem->dep_map, subclass, 0, _RET_IP_);
>
> LOCK_CONTENDED(sem, __down_write_trylock, __down_write);
> + rwsem_set_owner(sem);
> }
>
> EXPORT_SYMBOL(down_write_nested);
> diff --git a/lib/rwsem.c b/lib/rwsem.c
> index 1d6e6e8..1472ff3 100644
> --- a/lib/rwsem.c
> +++ b/lib/rwsem.c
> @@ -8,6 +8,7 @@
> */
> #include <linux/rwsem.h>
> #include <linux/sched.h>
> +#include <linux/sched/rt.h>
> #include <linux/init.h>
> #include <linux/export.h>
>
> @@ -27,6 +28,10 @@ void __init_rwsem(struct rw_semaphore *sem, const char *name,
> sem->count = RWSEM_UNLOCKED_VALUE;
> raw_spin_lock_init(&sem->wait_lock);
> INIT_LIST_HEAD(&sem->wait_list);
> +#ifdef CONFIG_RWSEM_SPIN_ON_WRITE_OWNER
> + sem->owner = NULL;
> + sem->spin_mlock = NULL;
> +#endif
> }
>
> EXPORT_SYMBOL(__init_rwsem);
> @@ -194,48 +199,252 @@ struct rw_semaphore __sched *rwsem_down_read_failed(struct rw_semaphore *sem)
> return sem;
> }
>
> +static inline int rwsem_try_write_lock(long count, struct rw_semaphore *sem)
> +{
> + if (!(count & RWSEM_ACTIVE_MASK)) {
> + /* Try acquiring the write lock. */
> + if (sem->count == RWSEM_WAITING_BIAS &&
> + cmpxchg(&sem->count, RWSEM_WAITING_BIAS,
> + RWSEM_ACTIVE_WRITE_BIAS) == RWSEM_WAITING_BIAS) {
> + if (!list_is_singular(&sem->wait_list))
> + rwsem_atomic_update(RWSEM_WAITING_BIAS, sem);
> + return 1;
> + }
> + }
> + return 0;
> +}
> +
> +#ifdef CONFIG_RWSEM_SPIN_ON_WRITE_OWNER
> +
> +struct mspin_node {
> + struct mspin_node *next ;
> + int locked; /* 1 if lock acquired */
> +};
> +#define MLOCK(rwsem) ((struct mspin_node **)&((rwsem)->spin_mlock))
> +
> +static noinline
> +void mspin_lock(struct mspin_node **lock, struct mspin_node *node)
> +{
> + struct mspin_node *prev;
> +
> + /* Init node */
> + node->locked = 0;
> + node->next = NULL;
> +
> + prev = xchg(lock, node);
> + if (likely(prev == NULL)) {
> + /* Lock acquired */
> + node->locked = 1;
> + return;
> + }
> + ACCESS_ONCE(prev->next) = node;
> + smp_wmb();
> + /* Wait until the lock holder passes the lock down */
> + while (!ACCESS_ONCE(node->locked))
> + arch_mutex_cpu_relax();
> +}
> +
> +static void mspin_unlock(struct mspin_node **lock, struct mspin_node *node)
> +{
> + struct mspin_node *next = ACCESS_ONCE(node->next);
> +
> + if (likely(!next)) {
> + /*
> + * Release the lock by setting it to NULL
> + */
> + if (cmpxchg(lock, node, NULL) == node)
> + return;
> + /* Wait until the next pointer is set */
> + while (!(next = ACCESS_ONCE(node->next)))
> + arch_mutex_cpu_relax();
> + }
> + ACCESS_ONCE(next->locked) = 1;
> + smp_wmb();
> +}
> +
> +static inline int rwsem_try_write_lock_unqueued(struct rw_semaphore *sem)
> +{
> + long count;
> +
> + count = ACCESS_ONCE(sem->count);
> +retry:
> + if (count == RWSEM_WAITING_BIAS) {
> + count = cmpxchg(&sem->count, RWSEM_WAITING_BIAS,
> + RWSEM_ACTIVE_WRITE_BIAS + RWSEM_WAITING_BIAS);
> + /* allow write lock stealing, try acquiring the write lock. */
> + if (count == RWSEM_WAITING_BIAS)
> + goto acquired;
> + else if (count == 0)
> + goto retry;
> + } else if (count == 0) {
> + count = cmpxchg(&sem->count, 0,
> + RWSEM_ACTIVE_WRITE_BIAS);
> + if (count == 0)
> + goto acquired;
> + else if (count == RWSEM_WAITING_BIAS)
> + goto retry;
> + }
> + return 0;
> +
> +acquired:
> + return 1;
> +}
> +
> +
> +static inline bool rwsem_can_spin_on_owner(struct rw_semaphore *sem)
> +{
> + int retval;
> + struct task_struct *owner;
> +
> + rcu_read_lock();
> + owner = ACCESS_ONCE(sem->owner);
> +
> + /* Spin only if active writer running */
> + if (owner)
> + retval = owner->on_cpu;
> + else
> + retval = false;
> +
> + rcu_read_unlock();
> + /*
> + * if lock->owner is not set, the sem owner may have just acquired
> + * it and not set the owner yet, or the sem has been released, or
> + * reader active.
> + */
> + return retval;
> +}
> +
> +static inline bool owner_running(struct rw_semaphore *lock,
> + struct task_struct *owner)
> +{
> + if (lock->owner != owner)
> + return false;
> +
> + /*
> + * Ensure we emit the owner->on_cpu, dereference _after_ checking
> + * lock->owner still matches owner, if that fails, owner might
> + * point to free()d memory, if it still matches, the rcu_read_lock()
> + * ensures the memory stays valid.
> + */
> + barrier();
> +
> + return owner->on_cpu;
> +}
> +
> +static noinline
> +int rwsem_spin_on_owner(struct rw_semaphore *lock, struct task_struct *owner)
> +{
> + rcu_read_lock();
> + while (owner_running(lock, owner)) {
> + if (need_resched())
> + break;
> +
> + arch_mutex_cpu_relax();
> + }
> + rcu_read_unlock();
> +
> + /*
> + * We break out the loop above on need_resched() and when the
> + * owner changed, which is a sign for heavy contention. Return
> + * success only when lock->owner is NULL.
> + */
> + return lock->owner == NULL;
> +}
> +
> +int rwsem_optimistic_spin(struct rw_semaphore *sem)
> +{
> + struct task_struct *owner;
> + int ret = 0;
> +
> + /* sem->wait_lock should not be held when doing optimistic spinning */
> + if (!rwsem_can_spin_on_owner(sem))
> + return ret;
> +
> + preempt_disable();
> + for (;;) {
> + struct mspin_node node;
> +
> + mspin_lock(MLOCK(sem), &node);
> + owner = ACCESS_ONCE(sem->owner);
> + if (owner && !rwsem_spin_on_owner(sem, owner)) {
> + mspin_unlock(MLOCK(sem), &node);
> + break;
> + }
> +
> + /* wait_lock will be acquired if write_lock is obtained */
> + if (rwsem_try_write_lock_unqueued(sem)) {
> + mspin_unlock(MLOCK(sem), &node);
> + ret = 1;
> + break;
> + }
> + mspin_unlock(MLOCK(sem), &node);
> +
> + /*
> + * When there's no owner, we might have preempted between the
> + * owner acquiring the lock and setting the owner field. If
> + * we're an RT task that will live-lock because we won't let
> + * the owner complete.
> + */
> + if (!owner && (need_resched() || rt_task(current)))
> + break;
> +
> + /*
> + * The cpu_relax() call is a compiler barrier which forces
> + * everything in this loop to be re-loaded. We don't need
> + * memory barriers as we'll eventually observe the right
> + * values at the cost of a few extra spins.
> + */
> + arch_mutex_cpu_relax();
> +
> + }
> +
> + preempt_enable();
> + return ret;
> +}
> +#endif
> +
> /*
> * wait until we successfully acquire the write lock
> */
> struct rw_semaphore __sched *rwsem_down_write_failed(struct rw_semaphore *sem)
> {
> - long count, adjustment = -RWSEM_ACTIVE_WRITE_BIAS;
> + long count;
> struct rwsem_waiter waiter;
> struct task_struct *tsk = current;
> + bool waiting = true;
>
> + count = rwsem_atomic_update(-RWSEM_ACTIVE_WRITE_BIAS, sem);
> +#ifdef CONFIG_RWSEM_SPIN_ON_WRITE_OWNER
> + /* do optimistic spinning */
> + if (rwsem_optimistic_spin(sem))
> + goto done;
> +#endif
> /* set up my own style of waitqueue */
> waiter.task = tsk;
> waiter.type = RWSEM_WAITING_FOR_WRITE;
>
> raw_spin_lock_irq(&sem->wait_lock);
> if (list_empty(&sem->wait_list))
> - adjustment += RWSEM_WAITING_BIAS;
> + waiting = false;
> list_add_tail(&waiter.list, &sem->wait_list);
>
> /* we're now waiting on the lock, but no longer actively locking */
> - count = rwsem_atomic_update(adjustment, sem);
> + if (waiting)
> + count = ACCESS_ONCE(sem->count);
> + else
> + count = rwsem_atomic_update(RWSEM_WAITING_BIAS, sem);
>
> /* If there were already threads queued before us and there are no
> * active writers, the lock must be read owned; so we try to wake
> * any read locks that were queued ahead of us. */
> - if (count > RWSEM_WAITING_BIAS &&
> - adjustment == -RWSEM_ACTIVE_WRITE_BIAS)
> + if ((count > RWSEM_WAITING_BIAS) && waiting)
> sem = __rwsem_do_wake(sem, RWSEM_WAKE_READERS);
>
> /* wait until we successfully acquire the lock */
> set_task_state(tsk, TASK_UNINTERRUPTIBLE);
> while (true) {
> - if (!(count & RWSEM_ACTIVE_MASK)) {
> - /* Try acquiring the write lock. */
> - count = RWSEM_ACTIVE_WRITE_BIAS;
> - if (!list_is_singular(&sem->wait_list))
> - count += RWSEM_WAITING_BIAS;
> -
> - if (sem->count == RWSEM_WAITING_BIAS &&
> - cmpxchg(&sem->count, RWSEM_WAITING_BIAS, count) ==
> - RWSEM_WAITING_BIAS)
> - break;
> - }
> + if (rwsem_try_write_lock(count, sem))
> + break;
>
> raw_spin_unlock_irq(&sem->wait_lock);
>
> @@ -250,6 +459,7 @@ struct rw_semaphore __sched *rwsem_down_write_failed(struct rw_semaphore *sem)
>
> list_del(&waiter.list);
> raw_spin_unlock_irq(&sem->wait_lock);
> +done:
> tsk->state = TASK_RUNNING;
>
> return sem;
>
>




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