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Date: Wed, 20 Jul 2016 14:37:34 -0400
From: Waiman Long <waiman.long@....com>
To: Jason Low <jason.low2@....com>
CC: <imre.deak@...el.com>, Peter Zijlstra <peterz@...radead.org>,
<linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...hat.com>,
Chris Wilson <chris@...is-wilson.co.uk>,
Daniel Vetter <daniel.vetter@...el.com>,
Davidlohr Bueso <dave@...olabs.net>, <jason.low2@...com>
Subject: Re: [RFC] Avoid mutex starvation when optimistic spinning is disabled
On 07/20/2016 12:39 AM, Jason Low wrote:
> On Tue, 2016-07-19 at 16:04 -0700, Jason Low wrote:
>> Hi Imre,
>>
>> Here is a patch which prevents a thread from spending too much "time"
>> waiting for a mutex in the !CONFIG_MUTEX_SPIN_ON_OWNER case.
>>
>> Would you like to try this out and see if this addresses the mutex
>> starvation issue you are seeing in your workload when optimistic
>> spinning is disabled?
> Although it looks like it didn't take care of the 'lock stealing' case
> in the slowpath. Here is the updated fixed version:
>
> ---
> Signed-off-by: Jason Low<jason.low2@....com>
> ---
> include/linux/mutex.h | 2 ++
> kernel/locking/mutex.c | 65 ++++++++++++++++++++++++++++++++++++++++++++------
> 2 files changed, 60 insertions(+), 7 deletions(-)
>
> diff --git a/include/linux/mutex.h b/include/linux/mutex.h
> index 2cb7531..c1ca68d 100644
> --- a/include/linux/mutex.h
> +++ b/include/linux/mutex.h
> @@ -57,6 +57,8 @@ struct mutex {
> #endif
> #ifdef CONFIG_MUTEX_SPIN_ON_OWNER
> struct optimistic_spin_queue osq; /* Spinner MCS lock */
> +#else
> + bool yield_to_waiter; /* Prevent starvation when spinning disabled */
> #endif
> #ifdef CONFIG_DEBUG_MUTEXES
> void *magic;
You don't need that on non-SMP system. So maybe you should put it under
#ifdef CONFIG_SMP block.
I actually have a similar boolean variable in my latest v4 mutex
patchset. We could probably merge them together.
> diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c
> index a70b90d..6c915ca 100644
> --- a/kernel/locking/mutex.c
> +++ b/kernel/locking/mutex.c
> @@ -55,6 +55,8 @@ __mutex_init(struct mutex *lock, const char *name, struct lock_class_key *key)
> mutex_clear_owner(lock);
> #ifdef CONFIG_MUTEX_SPIN_ON_OWNER
> osq_lock_init(&lock->osq);
> +#else
> + lock->yield_to_waiter = false;
> #endif
>
> debug_mutex_init(lock, name, key);
> @@ -71,6 +73,9 @@ EXPORT_SYMBOL(__mutex_init);
> */
> __visible void __sched __mutex_lock_slowpath(atomic_t *lock_count);
>
> +
> +static inline bool need_yield_to_waiter(struct mutex *lock);
> +
> /**
> * mutex_lock - acquire the mutex
> * @lock: the mutex to be acquired
> @@ -95,11 +100,15 @@ __visible void __sched __mutex_lock_slowpath(atomic_t *lock_count);
> void __sched mutex_lock(struct mutex *lock)
> {
> might_sleep();
> +
> /*
> * The locking fastpath is the 1->0 transition from
> * 'unlocked' into 'locked' state.
> */
> - __mutex_fastpath_lock(&lock->count, __mutex_lock_slowpath);
> + if (!need_yield_to_waiter(lock))
> + __mutex_fastpath_lock(&lock->count, __mutex_lock_slowpath);
> + else
> + __mutex_lock_slowpath(&lock->count);
> mutex_set_owner(lock);
> }
>
> @@ -398,12 +407,39 @@ done:
>
> return false;
> }
> +
> +static inline void do_yield_to_waiter(struct mutex *lock, int loops)
> +{
> + return;
> +}
> +
> +static inline bool need_yield_to_waiter(struct mutex *lock)
> +{
> + return false;
> +}
> +
> #else
> static bool mutex_optimistic_spin(struct mutex *lock,
> struct ww_acquire_ctx *ww_ctx, const bool use_ww_ctx)
> {
> return false;
> }
> +
> +#define MUTEX_MAX_WAIT 32
> +
> +static inline void do_yield_to_waiter(struct mutex *lock, int loops)
> +{
> + if (loops< MUTEX_MAX_WAIT)
> + return;
> +
> + if (lock->yield_to_waiter != true)
> + lock->yield_to_waiter =true;
> +}
> +
> +static inline bool need_yield_to_waiter(struct mutex *lock)
> +{
> + return lock->yield_to_waiter;
> +}
> #endif
>
> __visible __used noinline
> @@ -510,6 +546,7 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
> struct mutex_waiter waiter;
> unsigned long flags;
> int ret;
> + int loop = 0;
>
> if (use_ww_ctx) {
> struct ww_mutex *ww = container_of(lock, struct ww_mutex, base);
> @@ -532,7 +569,7 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
> * Once more, try to acquire the lock. Only try-lock the mutex if
> * it is unlocked to reduce unnecessary xchg() operations.
> */
> - if (!mutex_is_locked(lock)&&
> + if (!need_yield_to_waiter(lock)&& !mutex_is_locked(lock)&&
> (atomic_xchg_acquire(&lock->count, 0) == 1))
> goto skip_wait;
>
> @@ -546,6 +583,8 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
> lock_contended(&lock->dep_map, ip);
>
> for (;;) {
> + loop++;
> +
> /*
> * Lets try to take the lock again - this is needed even if
> * we get here for the first time (shortly after failing to
> @@ -556,7 +595,8 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
> * other waiters. We only attempt the xchg if the count is
> * non-negative in order to avoid unnecessary xchg operations:
> */
> - if (atomic_read(&lock->count)>= 0&&
> + if ((!need_yield_to_waiter(lock) || loop> 1)&&
> + atomic_read(&lock->count)>= 0&&
> (atomic_xchg_acquire(&lock->count, -1) == 1))
>
I think you need to reset the yield_to_waiter variable here when loop >
1 instead of at the end of the loop.
> break;
>
> @@ -581,6 +621,7 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
> spin_unlock_mutex(&lock->wait_lock, flags);
> schedule_preempt_disabled();
> spin_lock_mutex(&lock->wait_lock, flags);
> + do_yield_to_waiter(lock, loop);
> }
> __set_task_state(task, TASK_RUNNING);
>
> @@ -590,6 +631,10 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
> atomic_set(&lock->count, 0);
> debug_mutex_free_waiter(&waiter);
>
> +#ifndef CONFIG_MUTEX_SPIN_ON_OWNER
> + lock->yield_to_waiter = false;
> +#endif
> +
Maybe you should do the reset in an inline function instead.
> skip_wait:
> /* got the lock - cleanup and rejoice! */
> lock_acquired(&lock->dep_map, ip);
> @@ -789,10 +834,13 @@ __mutex_lock_interruptible_slowpath(struct mutex *lock);
> */
> int __sched mutex_lock_interruptible(struct mutex *lock)
> {
> - int ret;
> + int ret = 1;
>
> might_sleep();
> - ret = __mutex_fastpath_lock_retval(&lock->count);
> +
> + if (!need_yield_to_waiter(lock))
> + ret = __mutex_fastpath_lock_retval(&lock->count);
> +
> if (likely(!ret)) {
> mutex_set_owner(lock);
> return 0;
> @@ -804,10 +852,13 @@ EXPORT_SYMBOL(mutex_lock_interruptible);
>
> int __sched mutex_lock_killable(struct mutex *lock)
> {
> - int ret;
> + int ret = 1;
>
> might_sleep();
> - ret = __mutex_fastpath_lock_retval(&lock->count);
> +
> + if (!need_yield_to_waiter(lock))
> + ret = __mutex_fastpath_lock_retval(&lock->count);
> +
> if (likely(!ret)) {
> mutex_set_owner(lock);
> return 0;
Cheers,
Longman
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