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

[Xuan Zhuo <xuanzhuo@xxxxxxxxxxxxxxxxx>]

On Mon, 22 Jan 2024 12:16:27 +0800, Jason Wang <jasowang@xxxxxxxxxx> wrote:
> On Mon, Jan 22, 2024 at 12:00 PM Xuan Zhuo <xuanzhuo@xxxxxxxxxxxxxxxxx> wrote:
> >
> > On Mon, 22 Jan 2024 11:14:30 +0800, Jason Wang <jasowang@xxxxxxxxxx> wrote:
> > > On Mon, Jan 22, 2024 at 10:12 AM Zhu Yanjun <yanjun.zhu@xxxxxxxxx> 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@xxxxxxxxxxxxxxxxxxxxxxxxxx/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.


>
> 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
> > > >
> > >
> >
>