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SubjectRe: WARNING in __mmdrop

On 2019/7/26 下午9:47, Michael S. Tsirkin wrote:
> On Fri, Jul 26, 2019 at 08:53:18PM +0800, 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).
>>>> 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?
> Sorry, let me explain: guests (and tasks in general)
> can trigger activity that will
> make synchronize_rcu take a long time.

Yes, I get this.

> Thus blocking
> an mmu notifier until synchronize_rcu finishes
> is a bad idea.

The question is, MMU notifier are allowed to be blocked on
invalidate_range_start() which could be much slower than
synchronize_rcu() to finish.

Looking at amdgpu_mn_invalidate_range_start_gfx() which calls
amdgpu_mn_invalidate_node() which did:

                r = reservation_object_wait_timeout_rcu(bo->tbo.resv,
                        true, false, MAX_SCHEDULE_TIMEOUT);


>>> 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())
>> return;
>>         if (rcu_gp_is_expedited())
>> synchronize_rcu_expedited();
>> else
>> wait_rcu_gp(call_rcu);
>> }
>> EXPORT_SYMBOL_GPL(synchronize_rcu);
> An admin can force rcu to finish faster, trading
> interrupts for responsiveness.

Yes, so when set, all each synchronize_rcu() will go for

>>>> 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.
> Sorry I still don't see the point.
> synchronize_rcu doesn't normally do an IPI either.

Not the case of when rcu_expedited is set. This can just 100% make sure
there's 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.
>> Thanks
> Absolutely. Need to fix up the bugs first.



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