linux-kernel - Re: [RFC] Avoid mutex starvation when optimistic spinning is disabled

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Date:	Fri, 22 Jul 2016 14:01:18 -0400
From:	Waiman Long <waiman.long@....com>
To:	Jason Low <jason.low2@....com>
CC:	<imre.deak@...el.com>, Peter Zijlstra <peterz@...radead.org>,
	<linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...hat.com>,
	Chris Wilson <chris@...is-wilson.co.uk>,
	Daniel Vetter <daniel.vetter@...el.com>,
	Davidlohr Bueso <dave@...olabs.net>, <jason.low2@...com>
Subject: Re: [RFC] Avoid mutex starvation when optimistic spinning is disabled

On 07/21/2016 06:29 PM, Jason Low wrote:
> On Wed, 2016-07-20 at 14:37 -0400, Waiman Long wrote:
>> On 07/20/2016 12:39 AM, Jason Low wrote:
>>> On Tue, 2016-07-19 at 16:04 -0700, Jason Low wrote:
>>>> Hi Imre,
>>>>
>>>> Here is a patch which prevents a thread from spending too much "time"
>>>> waiting for a mutex in the !CONFIG_MUTEX_SPIN_ON_OWNER case.
>>>>
>>>> Would you like to try this out and see if this addresses the mutex
>>>> starvation issue you are seeing in your workload when optimistic
>>>> spinning is disabled?
>>> Although it looks like it didn't take care of the 'lock stealing' case
>>> in the slowpath. Here is the updated fixed version:
>>>
>>> ---
>>> Signed-off-by: Jason Low<jason.low2@....com>
>>> ---
>>>    include/linux/mutex.h  |  2 ++
>>>    kernel/locking/mutex.c | 65 ++++++++++++++++++++++++++++++++++++++++++++------
>>>    2 files changed, 60 insertions(+), 7 deletions(-)
>>>
>>> diff --git a/include/linux/mutex.h b/include/linux/mutex.h
>>> index 2cb7531..c1ca68d 100644
>>> --- a/include/linux/mutex.h
>>> +++ b/include/linux/mutex.h
>>> @@ -57,6 +57,8 @@ struct mutex {
>>>    #endif
>>>    #ifdef CONFIG_MUTEX_SPIN_ON_OWNER
>>>    	struct optimistic_spin_queue osq; /* Spinner MCS lock */
>>> +#else
>>> +	bool yield_to_waiter; /* Prevent starvation when spinning disabled */
>>>    #endif
>>>    #ifdef CONFIG_DEBUG_MUTEXES
>>>    	void			*magic;
>> You don't need that on non-SMP system. So maybe you should put it under
>> #ifdef CONFIG_SMP block.
> Right, maybe something like:
>
>      #ifdef CONFIG_MUTEX_SPIN_ON_OWNER
> 	...
> 	...
>      #elif !defined(CONFIG_SMP) /* If optimistic spinning disabled */
>          bool yield_to_waiter;
>      #endif
>
>>> @@ -556,7 +595,8 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
>>>    		 * other waiters. We only attempt the xchg if the count is
>>>    		 * non-negative in order to avoid unnecessary xchg operations:
>>>    		 */
>>> -		if (atomic_read(&lock->count)>= 0&&
>>> +		if ((!need_yield_to_waiter(lock) || loop>   1)&&
>>> +		    atomic_read(&lock->count)>= 0&&
>>>    		(atomic_xchg_acquire(&lock->count, -1) == 1))
>>>    	
>> I think you need to reset the yield_to_waiter variable here when loop>
>> 1 instead of at the end of the loop.
> So I think in the current state, only the top waiter would be able to
> both set and clear the yield_to_waiter variable anyway. However, I agree
> that this detail is not obvious and it would be better to reset the
> variable here when loop>  1 to make it more readable.

You should only reset the variable when loop > 1. You may also need to 
check in the error exit path as well.

Cheers,
Longman