netdev - Re: Re: [RFC v4 10/11] vduse: Introduce a workqueue for irq injection

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Date:   Fri, 5 Mar 2021 16:12:36 +0800
From:   Yongji Xie <xieyongji@...edance.com>
To:     Jason Wang <jasowang@...hat.com>
Cc:     "Michael S. Tsirkin" <mst@...hat.com>,
        Stefan Hajnoczi <stefanha@...hat.com>,
        Stefano Garzarella <sgarzare@...hat.com>,
        Parav Pandit <parav@...dia.com>, Bob Liu <bob.liu@...cle.com>,
        Christoph Hellwig <hch@...radead.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        Matthew Wilcox <willy@...radead.org>, viro@...iv.linux.org.uk,
        Jens Axboe <axboe@...nel.dk>, bcrl@...ck.org,
        Jonathan Corbet <corbet@....net>,
        virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org,
        kvm@...r.kernel.org, linux-aio@...ck.org,
        linux-fsdevel@...r.kernel.org
Subject: Re: Re: [RFC v4 10/11] vduse: Introduce a workqueue for irq injection

On Fri, Mar 5, 2021 at 3:37 PM Jason Wang <jasowang@...hat.com> wrote:
>
>
> On 2021/3/5 3:27 下午, Yongji Xie wrote:
> > On Fri, Mar 5, 2021 at 3:01 PM Jason Wang <jasowang@...hat.com> wrote:
> >>
> >> On 2021/3/5 2:36 下午, Yongji Xie wrote:
> >>> On Fri, Mar 5, 2021 at 11:42 AM Jason Wang <jasowang@...hat.com> wrote:
> >>>> On 2021/3/5 11:30 上午, Yongji Xie wrote:
> >>>>> On Fri, Mar 5, 2021 at 11:05 AM Jason Wang <jasowang@...hat.com> wrote:
> >>>>>> On 2021/3/4 4:58 下午, Yongji Xie wrote:
> >>>>>>> On Thu, Mar 4, 2021 at 2:59 PM Jason Wang <jasowang@...hat.com> wrote:
> >>>>>>>> On 2021/2/23 7:50 下午, Xie Yongji wrote:
> >>>>>>>>> This patch introduces a workqueue to support injecting
> >>>>>>>>> virtqueue's interrupt asynchronously. This is mainly
> >>>>>>>>> for performance considerations which makes sure the push()
> >>>>>>>>> and pop() for used vring can be asynchronous.
> >>>>>>>> Do you have pref numbers for this patch?
> >>>>>>>>
> >>>>>>> No, I can do some tests for it if needed.
> >>>>>>>
> >>>>>>> Another problem is the VIRTIO_RING_F_EVENT_IDX feature will be useless
> >>>>>>> if we call irq callback in ioctl context. Something like:
> >>>>>>>
> >>>>>>> virtqueue_push();
> >>>>>>> virtio_notify();
> >>>>>>>         ioctl()
> >>>>>>> -------------------------------------------------
> >>>>>>>             irq_cb()
> >>>>>>>                 virtqueue_get_buf()
> >>>>>>>
> >>>>>>> The used vring is always empty each time we call virtqueue_push() in
> >>>>>>> userspace. Not sure if it is what we expected.
> >>>>>> I'm not sure I get the issue.
> >>>>>>
> >>>>>> THe used ring should be filled by virtqueue_push() which is done by
> >>>>>> userspace before?
> >>>>>>
> >>>>> After userspace call virtqueue_push(), it always call virtio_notify()
> >>>>> immediately. In traditional VM (vhost-vdpa) cases, virtio_notify()
> >>>>> will inject an irq to VM and return, then vcpu thread will call
> >>>>> interrupt handler. But in container (virtio-vdpa) cases,
> >>>>> virtio_notify() will call interrupt handler directly. So it looks like
> >>>>> we have to optimize the virtio-vdpa cases. But one problem is we don't
> >>>>> know whether we are in the VM user case or container user case.
> >>>> Yes, but I still don't get why used ring is empty after the ioctl()?
> >>>> Used ring does not use bounce page so it should be visible to the kernel
> >>>> driver. What did I miss :) ?
> >>>>
> >>> Sorry, I'm not saying the kernel can't see the correct used vring. I
> >>> mean the kernel will consume the used vring in the ioctl context
> >>> directly in the virtio-vdpa case. In userspace's view, that means
> >>> virtqueue_push() is used vring's producer and virtio_notify() is used
> >>> vring's consumer. They will be called one by one in one thread rather
> >>> than different threads, which looks odd and has a bad effect on
> >>> performance.
> >>
> >> Yes, that's why we need a workqueue (WQ_UNBOUND you used). Or do you
> >> want to squash this patch into patch 8?
> >>
> >> So I think we can see obvious difference when virtio-vdpa is used.
> >>
> > But it looks like we don't need this workqueue in vhost-vdpa cases.
> > Any suggestions?
>
>
> I haven't had a deep thought. But I feel we can solve this by using the
> irq bypass manager (or something similar). Then we don't need it to be
> relayed via workqueue and vdpa. But I'm not sure how hard it will be.
>

 Or let vdpa bus drivers give us some information?

> Do you see any obvious performance regression by using the workqueue? Or
> we can optimize it in the future.
>

Agree.

Thanks,
Yongji