Re: [PATCH v2 1/4] locking/mutex: Add waiter parameter to mutex_optimistic_spin()

[Waiman Long <waiman.long@hpe.com>]

On 02/12/2016 05:02 PM, Davidlohr Bueso wrote:
> On Fri, 12 Feb 2016, Waiman Long wrote:
>
>> This patch adds a new waiter parameter to the mutex_optimistic_spin()
>> function to prepare it to be used by a waiter-spinner that doesn't
>> need to go into the OSQ as there can only be one waiter-spinner which
>> is the head of the waiting queue.
>>
>> Signed-off-by: Waiman Long <Waiman.Long@hp.com>
>> ---
>> kernel/locking/mutex.c |   55 
>> ++++++++++++++++++++++++++++++++---------------
>> 1 files changed, 37 insertions(+), 18 deletions(-)
>>
>> diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c
>> index 0551c21..3c41448 100644
>> --- a/kernel/locking/mutex.c
>> +++ b/kernel/locking/mutex.c
>> @@ -273,11 +273,15 @@ static inline int 
>> mutex_can_spin_on_owner(struct mutex *lock)
>>
>> /*
>>  * Atomically try to take the lock when it is available
>> + *
>> + * For waiter-spinner, the count needs to be set to -1 first which 
>> will be
>> + * cleared to 0 later on if the list becomes empty. For regular 
>> spinner,
>> + * the count will be set to 0.
>>  */
>> -static inline bool mutex_try_to_acquire(struct mutex *lock)
>> +static inline bool mutex_try_to_acquire(struct mutex *lock, int waiter)
>> {
>>     return !mutex_is_locked(lock) &&
>> -        (atomic_cmpxchg_acquire(&lock->count, 1, 0) == 1);
>> +        (atomic_cmpxchg_acquire(&lock->count, 1, waiter ? -1 : 0) == 
>> 1);
>> }
>
> This can be a really hot path, could we get rid of the waiter check 
> and just
> introduce mutex_tro_to_acquire_waiter() or such and set the counter to 
> -1 there?
>
> Thanks,
> Davidlohr

It is hot in the sense that the lock cacheline is highly contested. On 
x86, the ?: statement will most likely be translated to a cmov 
instruction before doing the cmpxchg. The cmov instruction won't affect 
the amount of cacheline contention on that lock cacheline. So I don't 
see there is any problem here.

Cheers,
Longman