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SubjectRe: WARNING in __mmdrop

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).

>> If we just worry about the IPI,
> With synchronize_rcu what I would worry about is that guest is stalled

Can this synchronize_rcu() be triggered by guest? If yes, there are
several other MMU notifiers that can block. Is vhost something special here?

> because system is busy because of other guests.
> With expedited it's the IPIs...

The current synchronize_rcu()  can force a expedited grace period:

void synchronize_rcu(void)
        if (rcu_blocking_is_gp())
        if (rcu_gp_is_expedited())

>> 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);
>>         }
>> ?
> Why would that be faster than synchronize_rcu?

No faster but no IPI.

>>> 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
> Sure, so we can do that for small rings.

Yes, that's possible but should be done on top.


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