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

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Date:   Tue, 27 Dec 2022 02:33:43 -0500
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Jason Wang <jasowang@...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

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.

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.
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?
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.

-- 
MST