Re: [PATCH] mutex: do not unnecessarily deal with waiters

[Ingo Molnar]

* Davidlohr Bueso <davidlohr.bueso@xxxxxx> wrote:

> From: Davidlohr Bueso <davidlohr.bueso@xxxxxx>
> 
> Upon entering the slowpath, we immediately attempt to acquire the lock
> by checking if it is already unlocked. If we are lucky enough that this
> is the case, then we don't need to deal with any waiter related logic.
> 
> Furthermore any checks for an empty wait_list are unnecessary as we
> already know that count is non-negative and hence no one is waiting for
> the lock.
> 
> Move the count check and xchg calls to be done before any waiters are
> setup - including waiter debugging. Upon failure to acquire the lock,
> the xchg sets the counter to 0, instead of -1 as it was originally.
> This can be done here since we set it back to -1 right at the beginning
> of the loop so other waiters are woken up when the lock is released.
> 
> When tested on a 8-socket (80 core) system against a vanilla 3.10-rc1
> kernel, this patch provides some small performance benefits (+2-6%).
> While it could be considered in the noise level, the average percentages
> were stable across multiple runs and no performance regressions were seen.
> Two big winners, for small amounts of users (10-100), were the short and
> compute workloads had a +19.36% and +%15.76% in jobs per minute.
>   
> Signed-off-by: Davidlohr Bueso <davidlohr.bueso@xxxxxx>
> ---
>  kernel/mutex.c | 26 ++++++++++++++------------
>  1 file changed, 14 insertions(+), 12 deletions(-)
> 
> diff --git a/kernel/mutex.c b/kernel/mutex.c
> index ad53a66..a8cd741 100644
> --- a/kernel/mutex.c
> +++ b/kernel/mutex.c
> @@ -306,7 +306,7 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
>  		owner = ACCESS_ONCE(lock->owner);
>  		if (owner && !mutex_spin_on_owner(lock, owner)) {
>  			mspin_unlock(MLOCK(lock), &node);
> -			break;
> +			goto slowpath;
>  		}
>  
>  		if ((atomic_read(&lock->count) == 1) &&
> @@ -314,8 +314,7 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
>  			lock_acquired(&lock->dep_map, ip);
>  			mutex_set_owner(lock);
>  			mspin_unlock(MLOCK(lock), &node);
> -			preempt_enable();
> -			return 0;
> +			goto done;
>  		}
>  		mspin_unlock(MLOCK(lock), &node);
>  
> @@ -326,7 +325,7 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
>  		 * the owner complete.
>  		 */
>  		if (!owner && (need_resched() || rt_task(task)))
> -			break;
> +			goto slowpath;
>  
>  		/*
>  		 * The cpu_relax() call is a compiler barrier which forces
> @@ -340,6 +339,14 @@ slowpath:
>  #endif
>  	spin_lock_mutex(&lock->wait_lock, flags);
>  
> +	/* once more, can we acquire the lock? */
> +	if (MUTEX_SHOW_NO_WAITER(lock) && (atomic_xchg(&lock->count, 0) == 1)) {
> +		lock_acquired(&lock->dep_map, ip);
> +		mutex_set_owner(lock);
> +		spin_unlock_mutex(&lock->wait_lock, flags);
> +		goto done;
> +	}
> +
>  	debug_mutex_lock_common(lock, &waiter);
>  	debug_mutex_add_waiter(lock, &waiter, task_thread_info(task));
>  
> @@ -347,9 +354,6 @@ slowpath:
>  	list_add_tail(&waiter.list, &lock->wait_list);
>  	waiter.task = task;
>  
> -	if (MUTEX_SHOW_NO_WAITER(lock) && (atomic_xchg(&lock->count, -1) == 1))
> -		goto done;
> -
>  	lock_contended(&lock->dep_map, ip);
>  
>  	for (;;) {
> @@ -363,7 +367,7 @@ slowpath:
>  		 * other waiters:
>  		 */
>  		if (MUTEX_SHOW_NO_WAITER(lock) &&
> -		   (atomic_xchg(&lock->count, -1) == 1))
> +		    (atomic_xchg(&lock->count, -1) == 1))
>  			break;
>  
>  		/*
> @@ -388,9 +392,8 @@ slowpath:
>  		spin_lock_mutex(&lock->wait_lock, flags);
>  	}
>  
> -done:
> +	/* got the lock - cleanup and rejoice! */
>  	lock_acquired(&lock->dep_map, ip);
> -	/* got the lock - rejoice! */
>  	mutex_remove_waiter(lock, &waiter, current_thread_info());
>  	mutex_set_owner(lock);
>  
> @@ -399,10 +402,9 @@ done:
>  		atomic_set(&lock->count, 0);
>  
>  	spin_unlock_mutex(&lock->wait_lock, flags);
> -
>  	debug_mutex_free_waiter(&waiter);
> +done:
>  	preempt_enable();
> -
>  	return 0;
>  }

So I tried this out yesterday, but it interacted with the Wait/Wound 
patches in tip:core/mutexes.

Maarten Lankhorst pointed out that if this patch is applied on top of the 
WW patches as-is, then we get this semantic merge conflict:

> > @@ -340,6 +339,14 @@ slowpath:
> >  #endif
> >     spin_lock_mutex(&lock->wait_lock, flags);
> >  
> > +   /* once more, can we acquire the lock? */
> > +   if (MUTEX_SHOW_NO_WAITER(lock) && (atomic_xchg(&lock->count, 0) == 1)) {
> > +           lock_acquired(&lock->dep_map, ip);
> > +           mutex_set_owner(lock);
> > +           spin_unlock_mutex(&lock->wait_lock, flags);
> > +           goto done;
> > +   }
> >
> ^^^^^^^^^^^^^^
>
> This part skips the whole if (!__builtin_constant_p(ww_ctx == NULL)) { 
> section with the wait_lock held.

Mind resolving that and merging this patch on top of the latest tip:master 
tree? Please also keep Maarten Cc:-ed.

Thanks,

	Ingo
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