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

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Date:	Tue, 16 Apr 2013 11:10:27 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Waiman Long <Waiman.Long@...com>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	David Howells <dhowells@...hat.com>,
	Dave Jones <davej@...hat.com>,
	Clark Williams <williams@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org, x86@...nel.org,
	linux-arch@...r.kernel.org,
	"Chandramouleeswaran, Aswin" <aswin@...com>,
	Davidlohr Bueso <davidlohr.bueso@...com>,
	"Norton, Scott J" <scott.norton@...com>,
	Rik van Riel <riel@...hat.com>
Subject: Re: [PATCH v2 2/3] mutex: Queue mutex spinners with MCS lock to
 reduce cacheline contention


* Waiman Long <Waiman.Long@...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'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.

Thanks,

	Ingo
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