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Date: Fri, 22 Jan 2016 08:46:30 -0800
From: Davidlohr Bueso <dave@stgolabs.net>
To: Waiman Long <waiman.long@hpe.com>
Cc: Ding Tianhong <dingtianhong@huawei.com>,
        Peter Zijlstra <peterz@infradead.org>, Ingo Molnar <mingo@redhat.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        "Paul E. McKenney" <paulmck@us.ibm.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Will Deacon <Will.Deacon@arm.com>, Jason Low <jason.low2@hp.com>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        Waiman Long <Waiman.Long@hp.com>
Subject: Re: [PATCH RFC] locking/mutexes: don't spin on owner when wait list
 is not NULL.
Message-ID: <20160122164630.GA1633@linux-uzut.site>
References: <56A0A4ED.3070308@huawei.com>
 <56A1638A.7050202@hpe.com>
 <20160122060944.GA28537@linux-uzut.site>
 <56A230C3.4070801@hpe.com>
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On Fri, 22 Jan 2016, Waiman Long wrote:

>The patch that I sent out is just a proof of concept to make sure
>that it can fix that particular case. I do plan to refactor it if I
>decide to go ahead with an official one. Unlike the OSQ, there can be
>no more than one waiter spinner as the wakeup function is directed to
>only the first task in the wait list and the spinning won't happen
>until the task is first woken up. In the worst case scenario, there
>are only 2 spinners spinning on the lock and the owner field, one
>from OSQ and one from the wait list. That shouldn't put too much
>cacheline contention traffic to the system.

Similarly, I guess we should also wakeup the next waiter in line after
releasing the wait_lock via wake_q. This would allow the woken waiter a
slightly better chance of finding the wait_lock free when continuing to
take the mutex.

Thanks,
Davidlohr