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Subject: Re: WARNING in __mmdrop
To:     "Michael S. Tsirkin" <mst@redhat.com>
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From:   Jason Wang <jasowang@redhat.com>
Message-ID: <5cc94f15-b229-a290-55f3-8295266edb2b@redhat.com>
Date:   Fri, 26 Jul 2019 20:00:58 +0800
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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.

If we just worry about the IPI, can we do something like in 
vhost_invalidate_vq_start()?

         if (map) {
                 /* In order to avoid possible IPIs with
                  * synchronize_rcu_expedited() we use call_rcu() +
                  * completion.
*/
init_completion(&c.completion);
                 call_rcu(&c.rcu_head, vhost_finish_vq_invalidation);
wait_for_completion(&c.completion);
                 vhost_set_map_dirty(vq, map, index);
vhost_map_unprefetch(map);
         }

?


> There's one other thing that bothers me, and that is that
> for large rings which are not physically contiguous
> we don't implement the optimization.
>
> For sure, that can wait, but I think eventually we should
> vmap large rings.


Yes, worth to try. But using direct map has its own advantage: it can 
use hugepage that vmap can't

Thanks