On Fri, Mar 5, 2021 at 3:37 PM Jason Wang <jasowang@xxxxxxxxxx> wrote: > > > On 2021/3/5 3:27 下午, Yongji Xie wrote: > > On Fri, Mar 5, 2021 at 3:01 PM Jason Wang <jasowang@xxxxxxxxxx> wrote: > >> > >> On 2021/3/5 2:36 下午, Yongji Xie wrote: > >>> On Fri, Mar 5, 2021 at 11:42 AM Jason Wang <jasowang@xxxxxxxxxx> wrote: > >>>> On 2021/3/5 11:30 上午, Yongji Xie wrote: > >>>>> On Fri, Mar 5, 2021 at 11:05 AM Jason Wang <jasowang@xxxxxxxxxx> wrote: > >>>>>> On 2021/3/4 4:58 下午, Yongji Xie wrote: > >>>>>>> On Thu, Mar 4, 2021 at 2:59 PM Jason Wang <jasowang@xxxxxxxxxx> 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
