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Date: Thu, 27 Jun 2013 11:00:16 +0200
From: Ingo Molnar <mingo@...nel.org>
To: Davidlohr Bueso <davidlohr.bueso@...com>,
Maarten Lankhorst <maarten.lankhorst@...onical.com>
Cc: Rik van Riel <riel@...hat.com>,
LKML <linux-kernel@...r.kernel.org>,
Peter Zijlstra <a.p.zijlstra@...llo.nl>,
Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] mutex: do not unnecessarily deal with waiters
* Davidlohr Bueso <davidlohr.bueso@...com> wrote:
> From: Davidlohr Bueso <davidlohr.bueso@...com>
>
> 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@...com>
> ---
> 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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