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

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Message-ID: <516D5F97.9050203@hp.com>
Date:	Tue, 16 Apr 2013 10:26:31 -0400
From:	Waiman Long <Waiman.Long@...com>
To:	Ingo Molnar <mingo@...nel.org>
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

On 04/16/2013 05:10 AM, Ingo Molnar wrote:
> * 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 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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