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

[Davidlohr Bueso]

ping, Ingo?

On Thu, 2013-05-30 at 18:12 -0700, Davidlohr Bueso wrote:
> ping?
> 
> On Thu, 2013-05-23 at 16:59 -0700, Davidlohr Bueso 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;
> >  }
> >  
> 
> 
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