netdev - Re: [PATCH 3/4] virtio_ring: introduce a per virtqueue waitqueue

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Date:   Wed, 28 Dec 2022 14:34:20 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     davem@...emloft.net, edumazet@...gle.com, kuba@...nel.org,
        pabeni@...hat.com, virtualization@...ts.linux-foundation.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        maxime.coquelin@...hat.com, alvaro.karsz@...id-run.com,
        eperezma@...hat.com
Subject: Re: [PATCH 3/4] virtio_ring: introduce a per virtqueue waitqueue


在 2022/12/27 17:38, Michael S. Tsirkin 写道:
> On Tue, Dec 27, 2022 at 05:12:58PM +0800, Jason Wang wrote:
>> 在 2022/12/27 15:33, Michael S. Tsirkin 写道:
>>> On Tue, Dec 27, 2022 at 12:30:35PM +0800, Jason Wang wrote:
>>>>> But device is still going and will later use the buffers.
>>>>>
>>>>> Same for timeout really.
>>>> Avoiding infinite wait/poll is one of the goals, another is to sleep.
>>>> If we think the timeout is hard, we can start from the wait.
>>>>
>>>> Thanks
>>> If the goal is to avoid disrupting traffic while CVQ is in use,
>>> that sounds more reasonable. E.g. someone is turning on promisc,
>>> a spike in CPU usage might be unwelcome.
>>
>> Yes, this would be more obvious is UP is used.
>>
>>
>>> things we should be careful to address then:
>>> 1- debugging. Currently it's easy to see a warning if CPU is stuck
>>>      in a loop for a while, and we also get a backtrace.
>>>      E.g. with this - how do we know who has the RTNL?
>>>      We need to integrate with kernel/watchdog.c for good results
>>>      and to make sure policy is consistent.
>>
>> That's fine, will consider this.
>>
>>
>>> 2- overhead. In a very common scenario when device is in hypervisor,
>>>      programming timers etc has a very high overhead, at bootup
>>>      lots of CVQ commands are run and slowing boot down is not nice.
>>>      let's poll for a bit before waiting?
>>
>> Then we go back to the question of choosing a good timeout for poll. And
>> poll seems problematic in the case of UP, scheduler might not have the
>> chance to run.
> Poll just a bit :) Seriously I don't know, but at least check once
> after kick.


I think it is what the current code did where the condition will be 
check before trying to sleep in the wait_event().


>
>>> 3- suprise removal. need to wake up thread in some way. what about
>>>      other cases of device breakage - is there a chance this
>>>      introduces new bugs around that? at least enumerate them please.
>>
>> The current code did:
>>
>> 1) check for vq->broken
>> 2) wakeup during BAD_RING()
>>
>> So we won't end up with a never woke up process which should be fine.
>>
>> Thanks
>
> BTW BAD_RING on removal will trigger dev_err. Not sure that is a good
> idea - can cause crashes if kernel panics on error.


Yes, it's better to use __virtqueue_break() instead.

But consider we will start from a wait first, I will limit the changes 
in virtio-net without bothering virtio core.

Thanks


>
>>>