Re: [PATCH v2 2/5] mutex: Modify the way optimistic spinners are queued

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Tue, Jan 28, 2014 at 12:23:34PM -0800, Paul E. McKenney wrote:
> On Tue, Jan 28, 2014 at 11:13:13AM -0800, Jason Low wrote:
> > The mutex->spin_mlock was introduced in order to ensure that only 1 thread
> > spins for lock acquisition at a time to reduce cache line contention. When
> > lock->owner is NULL and the lock->count is still not 1, the spinner(s) will
> > continually release and obtain the lock->spin_mlock. This can generate
> > quite a bit of overhead/contention, and also might just delay the spinner
> > from getting the lock.
> > 
> > This patch modifies the way optimistic spinners are queued by queuing before
> > entering the optimistic spinning loop as oppose to acquiring before every
> > call to mutex_spin_on_owner(). So in situations where the spinner requires
> > a few extra spins before obtaining the lock, then there will only be 1 spinner
> > trying to get the lock and it will avoid the overhead from unnecessarily
> > unlocking and locking the spin_mlock.
> > 
> > Signed-off-by: Jason Low <jason.low2@hp.com>
> 
> One question below.  Also, this might well have a visible effect on
> performance, so would be good to see the numbers.

Never mind, I see the numbers in your patch 0.  :-/

							Thanx, Paul

> > ---
> >  kernel/locking/mutex.c |   16 +++++++---------
> >  1 files changed, 7 insertions(+), 9 deletions(-)
> > 
> > diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c
> > index 85c6be1..7519d27 100644
> > --- a/kernel/locking/mutex.c
> > +++ b/kernel/locking/mutex.c
> > @@ -419,6 +419,7 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
> >  	struct mutex_waiter waiter;
> >  	unsigned long flags;
> >  	int ret;
> > +	struct mspin_node node;
> > 
> >  	preempt_disable();
> >  	mutex_acquire_nest(&lock->dep_map, subclass, 0, nest_lock, ip);
> > @@ -449,9 +450,9 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
> >  	if (!mutex_can_spin_on_owner(lock))
> >  		goto slowpath;
> > 
> > +	mspin_lock(MLOCK(lock), &node);
> >  	for (;;) {
> >  		struct task_struct *owner;
> > -		struct mspin_node  node;
> > 
> >  		if (use_ww_ctx && ww_ctx->acquired > 0) {
> >  			struct ww_mutex *ww;
> > @@ -466,19 +467,16 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
> >  			 * performed the optimistic spinning cannot be done.
> >  			 */
> >  			if (ACCESS_ONCE(ww->ctx))
> > -				goto slowpath;
> > +				break;
> >  		}
> > 
> >  		/*
> >  		 * If there's an owner, wait for it to either
> >  		 * release the lock or go to sleep.
> >  		 */
> > -		mspin_lock(MLOCK(lock), &node);
> >  		owner = ACCESS_ONCE(lock->owner);
> > -		if (owner && !mutex_spin_on_owner(lock, owner)) {
> > -			mspin_unlock(MLOCK(lock), &node);
> > -			goto slowpath;
> > -		}
> > +		if (owner && !mutex_spin_on_owner(lock, owner))
> > +			break;
> > 
> >  		if ((atomic_read(&lock->count) == 1) &&
> >  		    (atomic_cmpxchg(&lock->count, 1, 0) == 1)) {
> > @@ -495,7 +493,6 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
> >  			preempt_enable();
> >  			return 0;
> >  		}
> > -		mspin_unlock(MLOCK(lock), &node);
> > 
> >  		/*
> >  		 * When there's no owner, we might have preempted between the
> > @@ -504,7 +501,7 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
> >  		 * the owner complete.
> >  		 */
> >  		if (!owner && (need_resched() || rt_task(task)))
> > -			goto slowpath;
> > +			break;
> > 
> >  		/*
> >  		 * The cpu_relax() call is a compiler barrier which forces
> > @@ -514,6 +511,7 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
> >  		 */
> >  		arch_mutex_cpu_relax();
> >  	}
> > +	mspin_unlock(MLOCK(lock), &node);
> >  slowpath:
> 
> Are there any remaining goto statements to slowpath?  If so, they need
> to release the lock.  If not, this label should be removed.
> 
> >  #endif
> >  	spin_lock_mutex(&lock->wait_lock, flags);
> > -- 
> > 1.7.1
> >