Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
MIME-Version: 1.0
References: <20230720083839.481487-1-jasowang@redhat.com> <20230720083839.481487-3-jasowang@redhat.com>
 <e4eb0162-d303-b17c-a71d-ca3929380b31@amd.com> <20230720170001-mutt-send-email-mst@kernel.org>
 <263a5ad7-1189-3be3-70de-c38a685bebe0@redhat.com> <20230721104445-mutt-send-email-mst@kernel.org>
 <6278a4aa-8901-b0e3-342f-5753a4bf32af@redhat.com> <20230721110925-mutt-send-email-mst@kernel.org>
 <e3490755-35ac-89b4-b0fa-b63720a9a5c9@redhat.com>
In-Reply-To: <e3490755-35ac-89b4-b0fa-b63720a9a5c9@redhat.com>
From:   Jason Wang <jasowang@redhat.com>
Date:   Mon, 24 Jul 2023 14:52:05 +0800
Message-ID: <CACGkMEv1B9xFE7-LrLQC3FbH6CxTZC+toHXoLHFvJWn6wgobrA@mail.gmail.com>
Subject: Re: [PATCH net-next v4 2/2] virtio-net: add cond_resched() to the
 command waiting loop
To:     Maxime Coquelin <maxime.coquelin@redhat.com>
Cc:     "Michael S. Tsirkin" <mst@redhat.com>,
        Shannon Nelson <shannon.nelson@amd.com>,
        xuanzhuo@linux.alibaba.com, netdev@vger.kernel.org,
        linux-kernel@vger.kernel.org,
        virtualization@lists.linux-foundation.org, edumazet@google.com,
        kuba@kernel.org, pabeni@redhat.com, davem@davemloft.net
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable
Precedence: bulk

On Sat, Jul 22, 2023 at 4:18=E2=80=AFAM Maxime Coquelin
<maxime.coquelin@redhat.com> wrote:
>
>
>
> On 7/21/23 17:10, Michael S. Tsirkin wrote:
> > On Fri, Jul 21, 2023 at 04:58:04PM +0200, Maxime Coquelin wrote:
> >>
> >>
> >> On 7/21/23 16:45, Michael S. Tsirkin wrote:
> >>> On Fri, Jul 21, 2023 at 04:37:00PM +0200, Maxime Coquelin wrote:
> >>>>
> >>>>
> >>>> On 7/20/23 23:02, Michael S. Tsirkin wrote:
> >>>>> On Thu, Jul 20, 2023 at 01:26:20PM -0700, Shannon Nelson wrote:
> >>>>>> On 7/20/23 1:38 AM, Jason Wang wrote:
> >>>>>>>
> >>>>>>> Adding cond_resched() to the command waiting loop for a better
> >>>>>>> co-operation with the scheduler. This allows to give CPU a breath=
 to
> >>>>>>> run other task(workqueue) instead of busy looping when preemption=
 is
> >>>>>>> not allowed on a device whose CVQ might be slow.
> >>>>>>>
> >>>>>>> Signed-off-by: Jason Wang <jasowang@redhat.com>
> >>>>>>
> >>>>>> This still leaves hung processes, but at least it doesn't pin the =
CPU any
> >>>>>> more.  Thanks.
> >>>>>> Reviewed-by: Shannon Nelson <shannon.nelson@amd.com>
> >>>>>>
> >>>>>
> >>>>> I'd like to see a full solution
> >>>>> 1- block until interrupt

I remember in previous versions, you worried about the extra MSI
vector. (Maybe I was wrong).

> >>>>
> >>>> Would it make sense to also have a timeout?
> >>>> And when timeout expires, set FAILED bit in device status?
> >>>
> >>> virtio spec does not set any limits on the timing of vq
> >>> processing.
> >>
> >> Indeed, but I thought the driver could decide it is too long for it.
> >>
> >> The issue is we keep waiting with rtnl locked, it can quickly make the
> >> system unusable.
> >
> > if this is a problem we should find a way not to keep rtnl
> > locked indefinitely.

Any ideas on this direction? Simply dropping rtnl during the busy loop
will result in a lot of races. This seems to require non-trivial
changes in the networking core.

>
>  From the tests I have done, I think it is. With OVS, a reconfiguration
> is performed when the VDUSE device is added, and when a MLX5 device is
> in the same bridge, it ends up doing an ioctl() that tries to take the
> rtnl lock. In this configuration, it is not possible to kill OVS because
> it is stuck trying to acquire rtnl lock for mlx5 that is held by virtio-
> net.

Yeah, basically, any RTNL users would be blocked forever.

And the infinite loop has other side effects like it blocks the freezer to =
work.

To summarize, there are three issues

1) busy polling
2) breaks freezer
3) hold RTNL during the loop

Solving 3 may help somehow for 2 e.g some pm routine e.g wireguard or
even virtnet_restore() itself may try to hold the lock.

>
> >
> >>>>> 2- still handle surprise removal correctly by waking in that case

This is basically what version 1 did?

https://lore.kernel.org/lkml/6026e801-6fda-fee9-a69b-d06a80368621@redhat.co=
m/t/

Thanks

> >>>>>
> >>>>>
> >>>>>
> >>>>>>> ---
> >>>>>>>      drivers/net/virtio_net.c | 4 +++-
> >>>>>>>      1 file changed, 3 insertions(+), 1 deletion(-)
> >>>>>>>
> >>>>>>> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> >>>>>>> index 9f3b1d6ac33d..e7533f29b219 100644
> >>>>>>> --- a/drivers/net/virtio_net.c
> >>>>>>> +++ b/drivers/net/virtio_net.c
> >>>>>>> @@ -2314,8 +2314,10 @@ static bool virtnet_send_command(struct vi=
rtnet_info *vi, u8 class, u8 cmd,
> >>>>>>>              * into the hypervisor, so the request should be hand=
led immediately.
> >>>>>>>              */
> >>>>>>>             while (!virtqueue_get_buf(vi->cvq, &tmp) &&
> >>>>>>> -              !virtqueue_is_broken(vi->cvq))
> >>>>>>> +              !virtqueue_is_broken(vi->cvq)) {
> >>>>>>> +               cond_resched();
> >>>>>>>                     cpu_relax();
> >>>>>>> +       }
> >>>>>>>
> >>>>>>>             return vi->ctrl->status =3D=3D VIRTIO_NET_OK;
> >>>>>>>      }
> >>>>>>> --
> >>>>>>> 2.39.3
> >>>>>>>
> >>>>>>> _______________________________________________
> >>>>>>> Virtualization mailing list
> >>>>>>> Virtualization@lists.linux-foundation.org
> >>>>>>> https://lists.linuxfoundation.org/mailman/listinfo/virtualization
> >>>>>
> >>>
> >
>