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From:   Jason Wang <jasowang@redhat.com>
Date:   Fri, 30 Dec 2022 11:43:08 +0800
Message-ID: <CACGkMEukqZX=6yz1yCj+psHp5c+ZGVVuEYTUssfRCTQZgVWS6g@mail.gmail.com>
Subject: Re: [PATCH 3/4] virtio_ring: introduce a per virtqueue waitqueue
To:     "Michael S. Tsirkin" <mst@redhat.com>
Cc:     davem@davemloft.net, edumazet@google.com, kuba@kernel.org,
        pabeni@redhat.com, virtualization@lists.linux-foundation.org,
        netdev@vger.kernel.org, linux-kernel@vger.kernel.org,
        maxime.coquelin@redhat.com, alvaro.karsz@solid-run.com,
        eperezma@redhat.com
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On Thu, Dec 29, 2022 at 4:10 PM Michael S. Tsirkin <mst@redhat.com> wrote:
>
> 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@redhat.com> wro=
te:
> > >
> > > 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@redhat.com> wr=
ote:
> > > > >
> > > > >
> > > > > =E5=9C=A8 2022/12/27 17:38, Michael S. Tsirkin =E5=86=99=E9=81=93=
:
> > > > > > On Tue, Dec 27, 2022 at 05:12:58PM +0800, Jason Wang wrote:
> > > > > >> =E5=9C=A8 2022/12/27 15:33, Michael S. Tsirkin =E5=86=99=E9=81=
=93:
> > > > > >>> 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 us=
e,
> > > > > >>> that sounds more reasonable. E.g. someone is turning on promi=
sc,
> > > > > >>> 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 res=
ults
> > > > > >>>      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 av=
oid
> > > > 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 appl=
y
> to this driver. The warning can be different.

Right, so it looks to me the only possible setup is the
watchdog_thres. I plan to trigger the warning every watchdog_thres * 2
second (as softlockup did).

And I think it would still make sense to fail, we can start with a
very long timeout like 1 minutes and break the device. Does this make
sense?

Thanks

>
>
> > >
> > > > And before the patch, we end up with a real infinite loop which cou=
ld
> > > > 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 hype=
rvisor,
> > > > > >>>      programming timers etc has a very high overhead, at boot=
up
> > > > > >>>      lots of CVQ commands are run and slowing boot down is no=
t 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 o=
nce
> > > > > > 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 b=
e 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 ch=
anges
> > > > > in virtio-net without bothering virtio core.
> > > > >
> > > > > Thanks
> > > > >
> > > > >
> > > > > >
> > > > > >>>
> > >
>