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From:   Jason Wang <jasowang@redhat.com>
Date:   Mon, 30 Jan 2023 10:53:54 +0800
Message-ID: <CACGkMEuya+_2P8d4hokoyL_LKGdVzyCC1nDwOCdZb0=+2rjKPQ@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 Sun, Jan 29, 2023 at 3:30 PM Michael S. Tsirkin <mst@redhat.com> wrote:
>
> On Sun, Jan 29, 2023 at 01:48:49PM +0800, Jason Wang wrote:
> > On Fri, Jan 27, 2023 at 6:35 PM Michael S. Tsirkin <mst@redhat.com> wro=
te:
> > >
> > > On Fri, Dec 30, 2022 at 11:43:08AM +0800, Jason Wang wrote:
> > > > 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> 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@redhat=
.com> wrote:
> > > > > > > > >
> > > > > > > > >
> > > > > > > > > =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 wr=
ote:
> > > > > > > > > >> =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 bu=
ffers.
> > > > > > > > > >>>>>
> > > > > > > > > >>>>> Same for timeout really.
> > > > > > > > > >>>> Avoiding infinite wait/poll is one of the goals, ano=
ther is to sleep.
> > > > > > > > > >>>> If we think the timeout is hard, we can start from t=
he 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 backtra=
ce.
> > > > > > > > > >>>      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 tri=
es 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 watch=
dog.
> > > > > >
> > > > > > 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 + time=
out +
> > > > > > 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.
> > > >
> > > > 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 ma=
ke
> > > > sense?
> > > >
> > > > Thanks
> > >
> > > I'd say we need to make this manageable then.
> >
> > Did you mean something like sysfs or module parameters?
>
> No I'd say pass it with an ioctl.
>
> > > Can't we do it normally
> > > e.g. react to an interrupt to return to userspace?
> >
> > I didn't get the meaning of this. Sorry.
> >
> > Thanks
>
> Standard way to handle things that can timeout and where userspace
> did not supply the time is to block until an interrupt
> then return EINTR.

Well this seems to be a huge change, ioctl(2) doesn't say it can
return EINTR now.

Actually, a driver timeout is used by other drivers when using
controlq/adminq (e.g i40e). Starting from a sane value (e.g 1 minutes
to avoid false negatives) seems to be a good first step.

> Userspace controls the timeout by
> using e.g. alarm(2).

Not used in iproute2 after a git grep.

Thanks

>
>
> > >
> > >
> > >
> > > > >
> > > > >
> > > > > > >
> > > > > > > > And before the patch, we end up with a real infinite loop w=
hich could
> > > > > > > > be caught by RCU stall detector which is not the case of th=
e 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 do=
wn is not nice.
> > > > > > > > > >>>      let's poll for a bit before waiting?
> > > > > > > > > >>
> > > > > > > > > >> Then we go back to the question of choosing a good tim=
eout for poll. And
> > > > > > > > > >> poll seems problematic in the case of UP, scheduler mi=
ght 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 conditi=
on will be
> > > > > > > > > check before trying to sleep in the wait_event().
> > > > > > > > >
> > > > > > > > >
> > > > > > > > > >
> > > > > > > > > >>> 3- suprise removal. need to wake up thread in some wa=
y. what about
> > > > > > > > > >>>      other cases of device breakage - is there a chan=
ce this
> > > > > > > > > >>>      introduces new bugs around that? at least enumer=
ate 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 limi=
t the changes
> > > > > > > > > in virtio-net without bothering virtio core.
> > > > > > > > >
> > > > > > > > > Thanks
> > > > > > > > >
> > > > > > > > >
> > > > > > > > > >
> > > > > > > > > >>>
> > > > > > >
> > > > >
> > >
>