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

[Waiman Long]

On 04/16/2013 05:10 AM, Ingo Molnar wrote:
> * Waiman Long<Waiman.Long@xxxxxx>  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
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