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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: <96b1d67c-3a8d-1224-e9f0-5f7725a3dc10@redhat.com>
Date:   Tue, 30 Jul 2019 15:44:47 +0800
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On 2019/7/29 下午10:44, Michael S. Tsirkin wrote:
> On Mon, Jul 29, 2019 at 10:24:43PM +0800, Jason Wang wrote:
>> On 2019/7/29 下午4:59, Michael S. Tsirkin wrote:
>>> On Mon, Jul 29, 2019 at 01:54:49PM +0800, Jason Wang wrote:
>>>> On 2019/7/26 下午9:49, Michael S. Tsirkin wrote:
>>>>>>> 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
>>>>> OK that sounds good. If we also set a flag to make
>>>>> vhost_exceeds_weight exit, then I think it will be all good.
>>>> After some experiments, I came up two methods:
>>>>
>>>> 1) switch to use vq->mutex, then we must take the vq lock during range
>>>> checking (but I don't see obvious slowdown for 16vcpus + 16queues). Setting
>>>> flags during weight check should work but it still can't address the worst
>>>> case: wait for the page to be swapped in. Is this acceptable?
>>>>
>>>> 2) using current RCU but replace synchronize_rcu() with vhost_work_flush().
>>>> The worst case is the same as 1) but we can check range without holding any
>>>> locks.
>>>>
>>>> Which one did you prefer?
>>>>
>>>> Thanks
>>> I would rather we start with 1 and switch to 2 after we
>>> can show some gain.
>>>
>>> But the worst case needs to be addressed.
>>
>> Yes.
>>
>>
>>> How about sending a signal to
>>> the vhost thread?  We will need to fix up error handling (I think that
>>> at the moment it will error out in that case, handling this as EFAULT -
>>> and we don't want to drop packets if we can help it, and surely not
>>> enter any error states.  In particular it might be especially tricky if
>>> we wrote into userspace memory and are now trying to log the write.
>>> I guess we can disable the optimization if log is enabled?).
>>
>> This may work but requires a lot of changes.
> I agree.
>
>> And actually it's the price of
>> using vq mutex.
> Not sure what's meant here.


I mean if we use vq mutex, it means the critical section was increased 
and we need to deal with swapping then.


>
>> Actually, the critical section should be rather small, e.g
>> just inside memory accessors.
> Also true.
>
>> I wonder whether or not just do synchronize our self like:
>>
>> static void inline vhost_inc_vq_ref(struct vhost_virtqueue *vq)
>> {
>>          int ref = READ_ONCE(vq->ref);
>>
>>          WRITE_ONCE(vq->ref, ref + 1);
>> smp_rmb();
>> }
>>
>> static void inline vhost_dec_vq_ref(struct vhost_virtqueue *vq)
>> {
>>          int ref = READ_ONCE(vq->ref);
>>
>> smp_wmb();
>>          WRITE_ONCE(vq->ref, ref - 1);
>> }
>>
>> static void inline vhost_wait_for_ref(struct vhost_virtqueue *vq)
>> {
>>          while (READ_ONCE(vq->ref));
>> mb();
>> }
> Looks good but I'd like to think of a strategy/existing lock that let us
> block properly as opposed to spinning, that would be more friendly to
> e.g. the realtime patch.


Does it make sense to disable preemption in the critical section? Then 
we don't need to block and we have a deterministic time spent on memory 
accssors?


>
>> Or using smp_load_acquire()/smp_store_release() instead?
>>
>> Thanks
> These are cheaper on x86, yes.


Will use this.

Thanks


>