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.l...@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
> >
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