Re: [PATCH v2 2/3] mutex: Queue mutex spinners with MCS lock to reduce cacheline contention

[Waiman Long <Waiman.Long@hp.com>]

On 04/16/2013 05:10 AM, Ingo Molnar wrote:
> * Waiman Long<Waiman.Long@hp.com>  wrote:
>
>> --- a/kernel/sched/core.c
>> +++ b/kernel/sched/core.c
>> @@ -3021,9 +3021,6 @@ static inline bool owner_running(struct mutex *lock, struct task_struct *owner)
>>    */
>>   int mutex_spin_on_owner(struct mutex *lock, struct task_struct *owner)
>>   {
>> -	if (!sched_feat(OWNER_SPIN))
>> -		return 0;
>> -
>>   	rcu_read_lock();
>>   	while (owner_running(lock, owner)) {
>>   		if (need_resched())
>> @@ -3040,6 +3037,27 @@ int mutex_spin_on_owner(struct mutex *lock, struct task_struct *owner)
>>   	 */
>>   	return lock->owner == NULL;
>>   }
>> +
>> +/*
>> + * Initial check for entering the mutex spinning loop
>> + */
>> +int mutex_can_spin_on_owner(struct mutex *lock)
>> +{
>> +	int retval = 1;
>> +
>> +	if (!sched_feat(OWNER_SPIN))
>> +		return 0;
>> +
>> +	rcu_read_lock();
>> +	if (lock->owner)
>> +		retval = lock->owner->on_cpu;
>> +	rcu_read_unlock();
>> +	/*
>> +	 * if lock->owner is not set, the mutex owner may have just acquired
>> +	 * it and not set the owner yet or the mutex has been released.
>> +	 */
>> +	return retval;
>> +}
> The SCHED_FEAT_OWNER_SPIN was really just an early hack we did to make
> with/without mutex-spinning testable.
I see.

> I'd suggest a preparatory patch that gets rid of that flag and moves these two
> functions from sched/core.c to mutex.c where they belong.
>
> This will also allow the removal of the mutex prototypes from sched.h.

Yes, I can certainly prepare a patch to remove SCHED_FEAT_OWNER_SPIN & 
move those functions back to mutex.c after my patch set goes in. As for 
the timing, do you want me to do it now or it can wait as I will start 
my vacation later this week and will be back by the end of the month.

Regards,
Longman