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

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Date:   Thu, 29 Dec 2022 03:10:38 -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 Thu, Dec 29, 2022 at 04:04:13PM +0800, Jason Wang wrote:
> On Thu, Dec 29, 2022 at 3:07 PM Michael S. Tsirkin <mst@...hat.com> wrote:
> >
> > On Wed, Dec 28, 2022 at 07:53:08PM +0800, Jason Wang wrote:
> > > On Wed, Dec 28, 2022 at 2:34 PM Jason Wang <jasowang@...hat.com> wrote:
> > > >
> > > >
> > > > 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.
> > >
> > > So after some investigation, it seems the watchdog.c doesn't help. The
> > > only export helper is touch_softlockup_watchdog() which tries to avoid
> > > triggering the lockups warning for the known slow path.
> >
> > I never said you can just use existing exporting APIs. You'll have to
> > write new ones :)
> 
> Ok, I thought you wanted to trigger similar warnings as a watchdog.
> 
> Btw, I wonder what kind of logic you want here. If we switch to using
> sleep, there won't be soft lockup anymore. A simple wait + timeout +
> warning seems sufficient?
> 
> Thanks

I'd like to avoid need to teach users new APIs. So watchdog setup to apply
to this driver. The warning can be different.


> >
> > > And before the patch, we end up with a real infinite loop which could
> > > be caught by RCU stall detector which is not the case of the sleep.
> > > What we can do is probably do a periodic netdev_err().
> > >
> > > Thanks
> >
> > Only with a bad device.
> >
> > > > >>
> > > > >>
> > > > >>> 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
> > > >
> > > >
> > > > >
> > > > >>>
> >