linux-kernel - Re: [PATCH] locking/mutex: Avoid spinner vs waiter starvation

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Date:	Fri, 12 Feb 2016 13:33:33 -0500
From:	Waiman Long <waiman.long@....com>
To:	Ding Tianhong <dingtianhong@...wei.com>
CC:	Davidlohr Bueso <dave@...olabs.net>,
	Peter Zijlstra <peterz@...radead.org>,
	Jason Low <jason.low2@...com>, Ingo Molnar <mingo@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"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] locking/mutex: Avoid spinner vs waiter starvation

On 02/03/2016 08:20 PM, Ding Tianhong wrote:
> On 2016/2/4 3:24, Davidlohr Bueso wrote:
>> On Wed, 03 Feb 2016, Ding Tianhong wrote:
>>
>>> Agree, but this patch is going to help the waiter in the wait list to get the lock, your scene probability looks more
>>> too low and I don't think it is a problem.
>> Sure, I was in fact implying its not the end of the world,
>> although it will be interesting to see the impact on different
>> (non pathological) workloads, even if it only affects a single
>> waiter. Also, technically this issue can also affect rwsems if
>> only using writers, but that's obviously pretty idiotic, so I
>> wouldn't worry about it.
>>
>> Thanks,
>> Davidlohr
>>
> Hi Davidlohr, Peter:
>
> According Davidlohr's suggestion, I use several VM to test Peter's patch, and sadly I found one VM
> still happen Hung Task for this problem, so I think we still need to think more about this solution.
>
> Thanks.
> Ding

After thinking about it, it may be the case the hung task missed the 
wakeup. If no new task enters the wait queue again, the hung task will 
never get woken up. Would you mind trying out my patch to see if you are 
still seeing hung task or not?

Thanks,
Longman

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