netdev - Re: [PATCH 1/1] virtio_net: Add timeout handler to avoid kernel hang

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Date: Mon, 22 Jan 2024 14:55:46 +0800
From: Jason Wang <jasowang@...hat.com>
To: Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
Cc: Andrew Lunn <andrew@...n.ch>, Heng Qi <hengqi@...ux.alibaba.com>, 
	Paolo Abeni <pabeni@...hat.com>, Zhu Yanjun <yanjun.zhu@...el.com>, mst@...hat.com, 
	davem@...emloft.net, edumazet@...gle.com, kuba@...nel.org, 
	virtualization@...ts.linux.dev, netdev@...r.kernel.org, 
	Zhu Yanjun <yanjun.zhu@...ux.dev>
Subject: Re: [PATCH 1/1] virtio_net: Add timeout handler to avoid kernel hang

On Mon, Jan 22, 2024 at 2:20 PM Xuan Zhuo <xuanzhuo@...ux.alibaba.com> wrote:
>
> On Mon, 22 Jan 2024 12:16:27 +0800, Jason Wang <jasowang@...hat.com> wrote:
> > On Mon, Jan 22, 2024 at 12:00 PM Xuan Zhuo <xuanzhuo@...ux.alibaba.com> wrote:
> > >
> > > On Mon, 22 Jan 2024 11:14:30 +0800, Jason Wang <jasowang@...hat.com> wrote:
> > > > On Mon, Jan 22, 2024 at 10:12 AM Zhu Yanjun <yanjun.zhu@...ux.dev> wrote:
> > > > >
> > > > >
> > > > > 在 2024/1/20 1:29, Andrew Lunn 写道:
> > > > > >>>>>        while (!virtqueue_get_buf(vi->cvq, &tmp) &&
> > > > > >>>>> -           !virtqueue_is_broken(vi->cvq))
> > > > > >>>>> +           !virtqueue_is_broken(vi->cvq)) {
> > > > > >>>>> +        if (timeout)
> > > > > >>>>> +            timeout--;
> > > > > >>>> This is not really a timeout, just a loop counter. 200 iterations could
> > > > > >>>> be a very short time on reasonable H/W. I guess this avoid the soft
> > > > > >>>> lockup, but possibly (likely?) breaks the functionality when we need to
> > > > > >>>> loop for some non negligible time.
> > > > > >>>>
> > > > > >>>> I fear we need a more complex solution, as mentioned by Micheal in the
> > > > > >>>> thread you quoted.
> > > > > >>> Got it. I also look forward to the more complex solution to this problem.
> > > > > >> Can we add a device capability (new feature bit) such as ctrq_wait_timeout
> > > > > >> to get a reasonable timeout？
> > > > > > The usual solution to this is include/linux/iopoll.h. If you can sleep
> > > > > > read_poll_timeout() otherwise read_poll_timeout_atomic().
> > > > >
> > > > > I read carefully the functions read_poll_timeout() and
> > > > > read_poll_timeout_atomic(). The timeout is set by the caller of the 2
> > > > > functions.
> > > >
> > > > FYI, in order to avoid a swtich of atomic or not, we need convert rx
> > > > mode setting to workqueue first:
> > > >
> > > > https://www.mail-archive.com/virtualization@lists.linux-foundation.org/msg60298.html
> > > >
> > > > >
> > > > > As such, can we add a module parameter to customize this timeout value
> > > > > by the user?
> > > >
> > > > Who is the "user" here, or how can the "user" know the value?
> > > >
> > > > >
> > > > > Or this timeout value is stored in device register, virtio_net driver
> > > > > will read this timeout value at initialization?
> > > >
> > > > See another thread. The design needs to be general, or you can post a RFC.
> > > >
> > > > In another thought, we've already had a tx watchdog, maybe we can have
> > > > something similar to cvq and use timeout + reset in that case.
> > >
> > > But we may block by the reset ^_^ if the device is broken?
> >
> > I mean vq reset here.
>
> I see.
>
> I mean when the deivce is broken, the vq reset also many be blocked.
>
>         void vp_modern_set_queue_reset(struct virtio_pci_modern_device *mdev, u16 index)
>         {
>                 struct virtio_pci_modern_common_cfg __iomem *cfg;
>
>                 cfg = (struct virtio_pci_modern_common_cfg __iomem *)mdev->common;
>
>                 vp_iowrite16(index, &cfg->cfg.queue_select);
>                 vp_iowrite16(1, &cfg->queue_reset);
>
>                 while (vp_ioread16(&cfg->queue_reset))
>                         msleep(1);
>
>                 while (vp_ioread16(&cfg->cfg.queue_enable))
>                         msleep(1);
>         }
>         EXPORT_SYMBOL_GPL(vp_modern_set_queue_reset);
>
> In this function, for the broken device, we can not expect something.

Yes, it's best effort, there's no guarantee then. But it doesn't harm to try.

Thanks

>
>
> >
> > It looks like we have multiple goals here
> >
> > 1) avoid lockups, using workqueue + cond_resched() seems to be
> > sufficient, it has issue but nothing new
> > 2) recover from the unresponsive device, the issue for timeout is that
> > it needs to deal with false positives
>
>
> I agree.
>
> But I want to add a new goal, cvq async. In the netdim, we will
> send many requests via the cvq, so the cvq async will be nice.
>
> Thanks.
>
>
> >
> > Thanks
> >
> > >
> > > Thanks.
> > >
> > >
> > > >
> > > > Thans
> > > >
> > > > >
> > > > > Zhu Yanjun
> > > > >
> > > > > >
> > > > > >       Andrew
> > > > >
> > > >
> > >
> >
>