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Subject: Re: WARNING in __mmdrop
From:   Jason Wang <jasowang@redhat.com>
To:     "Michael S. Tsirkin" <mst@redhat.com>
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Date:   Fri, 26 Jul 2019 21:36:18 +0800
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On 2019/7/26 下午8:53, Jason Wang wrote:
>
> On 2019/7/26 下午8:38, Michael S. Tsirkin wrote:
>> On Fri, Jul 26, 2019 at 08:00:58PM +0800, Jason Wang wrote:
>>> On 2019/7/26 下午7:49, Michael S. Tsirkin wrote:
>>>> On Thu, Jul 25, 2019 at 10:25:25PM +0800, Jason Wang wrote:
>>>>> On 2019/7/25 下午9:26, Michael S. Tsirkin wrote:
>>>>>>> Exactly, and that's the reason actually I use synchronize_rcu() 
>>>>>>> there.
>>>>>>>
>>>>>>> So the concern is still the possible synchronize_expedited()?
>>>>>> I think synchronize_srcu_expedited.
>>>>>>
>>>>>> synchronize_expedited sends lots of IPI and is bad for realtime VMs.
>>>>>>
>>>>>>> Can I do this
>>>>>>> on through another series on top of the incoming V2?
>>>>>>>
>>>>>>> Thanks
>>>>>>>
>>>>>> The question is this: is this still a gain if we switch to the
>>>>>> more expensive srcu? If yes then we can keep the feature on,
>>>>> I think we only care about the cost on srcu_read_lock() which 
>>>>> looks pretty
>>>>> tiny form my point of view. Which is basically a READ_ONCE() + 
>>>>> WRITE_ONCE().
>>>>>
>>>>> Of course I can benchmark to see the difference.
>>>>>
>>>>>
>>>>>> if not we'll put it off until next release and think
>>>>>> of better solutions. rcu->srcu is just a find and replace,
>>>>>> don't see why we need to defer that. can be a separate patch
>>>>>> for sure, but we need to know how well it works.
>>>>> I think I get here, let me try to do that in V2 and let's see the 
>>>>> numbers.
>>>>>
>>>>> Thanks
>>>
>>> It looks to me for tree rcu, its srcu_read_lock() have a mb() which 
>>> is too
>>> expensive for us.
>> I will try to ponder using vq lock in some way.
>> Maybe with trylock somehow ...
>
>
> Ok, let me retry if necessary (but I do remember I end up with 
> deadlocks last try). 


Ok, I play a little with this. And it works so far. Will do more testing 
tomorrow.

One reason could be I switch to use get_user_pages_fast() to 
__get_user_pages_fast() which doesn't need mmap_sem.

Thanks