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

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Message-ID: <d77bc1ce-b73f-1ba8-f04f-b3bffeb731c3@redhat.com>
Date:   Tue, 27 Dec 2022 17:12:58 +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 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.


> 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


>
>