Date:	Wed, 26 Jun 2013 15:40:07 -0700
From:	Davidlohr Bueso <davidlohr.bueso@...com>
To:	Tim Chen <tim.c.chen@...ux.intel.com>
Cc:	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Alex Shi <alex.shi@...el.com>,
	Andi Kleen <andi@...stfloor.org>,
	Michel Lespinasse <walken@...gle.com>,
	Matthew R Wilcox <matthew.r.wilcox@...el.com>,
	Dave Hansen <dave.hansen@...el.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Rik van Riel <riel@...hat.com>,
	Peter Hurley <peter@...leysoftware.com>,
	linux-kernel@...r.kernel.org, linux-mm <linux-mm@...ck.org>
Subject: Re: [PATCH v4 5/5] rwsem: do optimistic spinning for writer lock
 acquisition

On Wed, 2013-06-26 at 15:28 -0700, Tim Chen wrote:
> We want to add optimistic spinning to rwsems because we've noticed that
> the writer rwsem does not perform as well as mutexes. Tim noticed that
> for exim (mail server) workloads, when reverting commit 4fc3f1d6 and Davidlohr
> noticed it when converting the i_mmap_mutex to a rwsem in some aim7
> workloads. We've noticed that the biggest difference, in a nutshell, is
> when we fail to acquire a mutex in the fastpath, optimistic spinning
> comes in to play and we can avoid a large amount of unnecessary sleeping
> and wait queue overhead.
> 
> For rwsems on the other hand, upon entering the writer slowpath in
> rwsem_down_write_failed(), we just acquire the ->wait_lock, add
> ourselves to the wait_queue and blocking until we get the lock.
> 
> Reviewed-by: Peter Zijlstra <peterz@...radead.org>
> Reviewed-by: Peter Hurley <peter@...leysoftware.com>
> Signed-off-by: Tim Chen <tim.c.chen@...ux.intel.com>
> Signed-off-by: Davidlohr Bueso <davidlohr.bueso@...com>
> ---
>  include/linux/rwsem.h |    3 +
>  init/Kconfig          |    9 +++
>  kernel/rwsem.c        |   29 +++++++++-
>  lib/rwsem.c           |  150 +++++++++++++++++++++++++++++++++++++++++++++----
>  4 files changed, 179 insertions(+), 12 deletions(-)
> 
> diff --git a/include/linux/rwsem.h b/include/linux/rwsem.h
> index 0616ffe..0c5933b 100644
> --- a/include/linux/rwsem.h
> +++ b/include/linux/rwsem.h
> @@ -29,6 +29,9 @@ 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;
> +#endif
>  };
>  
>  extern struct rw_semaphore *rwsem_down_read_failed(struct rw_semaphore *sem);
> diff --git a/init/Kconfig b/init/Kconfig
> index 9d3a788..1c582d1 100644
> --- a/init/Kconfig
> +++ b/init/Kconfig
> @@ -1595,6 +1595,15 @@ 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.  This gives a greater chance for writer to
> +	  acquire a semaphore before blocking it and putting it to sleep.
> +

Quoting from kernel/mutex.c:

"The rationale is that if the lock owner is running, it is likely to
release the lock soon." 

It would be good to add that to the Kconfig.

>  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..b7fd8ff 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 +28,9 @@ 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;
> +#endif
>  }
>  
>  EXPORT_SYMBOL(__init_rwsem);
> @@ -194,6 +198,130 @@ struct rw_semaphore __sched *rwsem_down_read_failed(struct rw_semaphore *sem)
>  	return sem;
>  }
>  
> +static inline int rwsem_try_write_lock(long count, bool need_lock,
> +	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 (need_lock)
> +				raw_spin_lock_irq(&sem->wait_lock);
> +			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
> +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 (;;) {
> +		owner = ACCESS_ONCE(sem->owner);
> +		if (owner && !rwsem_spin_on_owner(sem, owner))
> +			break;
> +
> +		/* wait_lock will be acquired if write_lock is obtained */
> +		if (rwsem_try_write_lock(sem->count, true, sem)) {
> +			ret = 1;
> +			break;
> +		}
> +
> +		/*
> +		 * 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
>   */
> @@ -202,6 +330,9 @@ struct rw_semaphore __sched *rwsem_down_write_failed(struct rw_semaphore *sem)
>  	long count, adjustment = -RWSEM_ACTIVE_WRITE_BIAS;
>  	struct rwsem_waiter waiter;
>  	struct task_struct *tsk = current;
> +#ifdef CONFIG_RWSEM_SPIN_ON_WRITE_OWNER
> +	bool try_optimistic_spin = true;
> +#endif
>  
>  	/* set up my own style of waitqueue */
>  	waiter.task = tsk;
> @@ -225,20 +356,17 @@ struct rw_semaphore __sched *rwsem_down_write_failed(struct rw_semaphore *sem)
>  	/* 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, false, sem))
> +			break;
>  
>  		raw_spin_unlock_irq(&sem->wait_lock);
>  
> +#ifdef CONFIG_RWSEM_SPIN_ON_WRITE_OWNER
> +		/* do optimistic spinning */
> +		if (try_optimistic_spin && rwsem_optimistic_spin(sem))
> +			break;
> +		try_optimistic_spin = false;
> +#endif
>  		/* Block until there are no active lockers. */
>  		do {
>  			schedule();


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