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

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Message-ID: <CACycT3uZ2ZPjUwVZqzQPZ4ke=VrHCkfNvYagA-oxggPUEUi0Vg@mail.gmail.com>
Date:   Fri, 5 Mar 2021 14:36:51 +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 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.

Thanks,
Yongji