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

[Ding Tianhong <dingtianhong@huawei.com>]

On 2016/2/3 5:19, Davidlohr Bueso wrote:
> On Mon, 01 Feb 2016, Peter Zijlstra wrote:
> 
>> Subject: locking/mutex: Avoid spinner vs waiter starvation
>> From: Peter Zijlstra <peterz@infradead.org>
>> Date: Fri, 22 Jan 2016 12:06:53 +0100
>>
>> Ding Tianhong reported that under his load the optimistic spinners
>> would totally starve a task that ended up on the wait list.
>>
>> Fix this by ensuring the top waiter also partakes in the optimistic
>> spin queue.
>>
>> There are a few subtle differences between the assumed state of
>> regular optimistic spinners and those already on the wait list, which
>> result in the @acquired complication of the acquire path.
>>
>> Most notable are:
>>
>> - waiters are on the wait list and need to be taken off
>> - mutex_optimistic_spin() sets the lock->count to 0 on acquire
>>   even though there might be more tasks on the wait list.
> 
> Right, the main impact I see with these complications are that the
> window of when a waiter takes the lock via spinning and then acquires
> the wait_lock to remove itself from the list, will allow an unlock
> thread to set the lock as available in the fastpath which could in
> turn allow a third thread the steal the lock. With high contention,
> this window will be come obviously larger as we contend for the
> wait_lock.
> 
> CPU-0                                  CPU-1            CPU-3
> __mutex_lock_common              mutex_optimistic_spin
>   (->count now 0)
>             __mutex_fastpath_unlock
>             (->count now 1)                 __mutex_fastpath_lock
>                                       (stolen)
>                                                        
> spin_lock_mutex(&lock->wait_lock, flags);
> 
> But we've always been bad when it comes to counter and waiters.
> 

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.

Thanks
Ding


> Thanks,
> Davidlohr
> 
> .
>