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Message-ID: <20221227022255-mutt-send-email-mst@kernel.org>
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
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