linux-kernel - Re: [PATCH v2 1/4] locking/mutex: Add waiter parameter to mutex_optimistic

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Date:	Mon, 15 Feb 2016 22:30:34 -0500
From:	Waiman Long <waiman.long@....com>
To:	Jason Low <jason.low2@...com>
CC:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>, <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ding Tianhong <dingtianhong@...wei.com>,
	Jason Low <jason.low2@....com>,
	Davidlohr Bueso <dave@...olabs.net>,
	"Paul E. McKenney" <paulmck@...ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Will Deacon <Will.Deacon@....com>,
	Tim Chen <tim.c.chen@...ux.intel.com>,
	Waiman Long <Waiman.Long@...com>
Subject: Re: [PATCH v2 1/4] locking/mutex: Add waiter parameter to mutex_optimistic_spin()

On 02/15/2016 10:00 PM, Jason Low wrote:
> On Mon, 2016-02-15 at 18:55 -0500, Waiman Long wrote:
>> On 02/12/2016 03:40 PM, Peter Zijlstra wrote:
>>> On Fri, Feb 12, 2016 at 12:32:12PM -0500, Waiman Long wrote:
>>>> @@ -358,8 +373,8 @@ static bool mutex_optimistic_spin(struct mutex *lock,
>>>>    			}
>>>>
>>>>    			mutex_set_owner(lock);
>>>> -			osq_unlock(&lock->osq);
>>>> -			return true;
>>>> +			acquired = true;
>>>> +			break;
>>>>    		}
>>>>
>>>>    		/*
>>>> @@ -380,7 +395,10 @@ static bool mutex_optimistic_spin(struct mutex *lock,
>>>>    		cpu_relax_lowlatency();
>>>>    	}
>>>>
>>>> -	osq_unlock(&lock->osq);
>>>> +	if (!waiter)
>>>> +		osq_unlock(&lock->osq);
>>>> +	if (acquired || waiter)
>>>> +		return acquired;
>>>>    done:
>>>>    	/*
>>>>    	 * If we fell out of the spin path because of need_resched(),
>>> Is there a reason to not also preempt in the wait-loop? Surely the same
>>> reason is still valid there too?
>> The waiter does check for need_sched(). So it will break out of the loop
>> and return false in this case. This causes the waiter to loop back and
>> goes to sleep if the lock can't be acquired. That is why I don't think
>> we need to do another schedule_preempt_disabled() here.
> The purpose of the additional reschedule point is to avoid delaying
> preemption, which still applies if the spinner is a waiter. If it is a
> waiter, the difference is that the delay isn't as long since it doesn't
> need to be added to the wait_list. Nonetheless, preemption delays can
> still occur, so I think the additional preemption point should also be
> there in the waiter case.

You are right. Taking the wait lock can introduce arbitrary delay. So I 
will modify the patch to fall through and check for preemption.

Cheers,
Longman