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

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Message-ID: <e933ec33-9d47-0ef5-9152-25cedd330ce2@redhat.com>
Date:   Fri, 5 Mar 2021 15:01:32 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Yongji Xie <xieyongji@...edance.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: [RFC v4 10/11] vduse: Introduce a workqueue for irq injection


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.

Thanks


>
> Thanks,
> Yongji
>