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

[Jason Wang]

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


> > >