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    SubjectRe: [PATCH net-next v2 3/5] virtio_ring: add packed ring support
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    On 2018/11/8 下午11:56, Michael S. Tsirkin wrote:
    > On Thu, Nov 08, 2018 at 07:51:48PM +0800, Tiwei Bie wrote:
    >> On Thu, Nov 08, 2018 at 04:18:25PM +0800, Jason Wang wrote:
    >>> On 2018/11/8 上午9:38, Tiwei Bie wrote:
    >>>>>> +
    >>>>>> + if (vq->vq.num_free < descs_used) {
    >>>>>> + pr_debug("Can't add buf len %i - avail = %i\n",
    >>>>>> + descs_used, vq->vq.num_free);
    >>>>>> + /* FIXME: for historical reasons, we force a notify here if
    >>>>>> + * there are outgoing parts to the buffer. Presumably the
    >>>>>> + * host should service the ring ASAP. */
    >>>>> I don't think we have a reason to do this for packed ring.
    >>>>> No historical baggage there, right?
    >>>> Based on the original commit log, it seems that the notify here
    >>>> is just an "optimization". But I don't quite understand what does
    >>>> the "the heuristics which KVM uses" refer to. If it's safe to drop
    >>>> this in packed ring, I'd like to do it.
    >>>
    >>> According to the commit log, it seems like a workaround of lguest networking
    >>> backend.
    >> Do you know why removing this notify in Tx will break "the
    >> heuristics which KVM uses"? Or what does "the heuristics
    >> which KVM uses" refer to?
    > Yes. QEMU has a mode where it disables notifications and processes TX
    > ring periodically from a timer. It's off by default but used to be on
    > by default a long time ago. If ring becomes full this causes traffic
    > stalls.


    Do you mean tx-timer? If yes, we can still enable it for packed ring and
    the timer will finally fired and we can go.


    > As a work-around Rusty put in this hack to kick on ring full
    > even with notifications disabled.


    From the commit log it looks more like a performance workaround instead
    of a bug fix.


    > It's easy enough to make sure QEMU
    > does not combine devices with packed ring support with the timer hack.
    > And I am guessing it's safe enough to also block that option completely
    > e.g. when virtio 1.0 is enabled.


    I agree.

    Thanks


    >>> I agree to drop it, we should not have such burden.
    >>>
    >>> But we should notice that, with this removed, the compare between packed vs
    >>> split is kind of unfair. Consider the removal of lguest support recently,
    >>> maybe we can drop this for split ring as well?
    >>>
    >>> Thanks
    >>>
    >>>
    >>>> commit 44653eae1407f79dff6f52fcf594ae84cb165ec4
    >>>> Author: Rusty Russell<rusty@rustcorp.com.au>
    >>>> Date: Fri Jul 25 12:06:04 2008 -0500
    >>>>
    >>>> virtio: don't always force a notification when ring is full
    >>>> We force notification when the ring is full, even if the host has
    >>>> indicated it doesn't want to know. This seemed like a good idea at
    >>>> the time: if we fill the transmit ring, we should tell the host
    >>>> immediately.
    >>>> Unfortunately this logic also applies to the receiving ring, which is
    >>>> refilled constantly. We should introduce real notification thesholds
    >>>> to replace this logic. Meanwhile, removing the logic altogether breaks
    >>>> the heuristics which KVM uses, so we use a hack: only notify if there are
    >>>> outgoing parts of the new buffer.
    >>>> Here are the number of exits with lguest's crappy network implementation:
    >>>> Before:
    >>>> network xmit 7859051 recv 236420
    >>>> After:
    >>>> network xmit 7858610 recv 118136
    >>>> Signed-off-by: Rusty Russell<rusty@rustcorp.com.au>
    >>>>
    >>>> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
    >>>> index 72bf8bc09014..21d9a62767af 100644
    >>>> --- a/drivers/virtio/virtio_ring.c
    >>>> +++ b/drivers/virtio/virtio_ring.c
    >>>> @@ -87,8 +87,11 @@ static int vring_add_buf(struct virtqueue *_vq,
    >>>> if (vq->num_free < out + in) {
    >>>> pr_debug("Can't add buf len %i - avail = %i\n",
    >>>> out + in, vq->num_free);
    >>>> - /* We notify*even if* VRING_USED_F_NO_NOTIFY is set here. */
    >>>> - vq->notify(&vq->vq);
    >>>> + /* FIXME: for historical reasons, we force a notify here if
    >>>> + * there are outgoing parts to the buffer. Presumably the
    >>>> + * host should service the ring ASAP. */
    >>>> + if (out)
    >>>> + vq->notify(&vq->vq);
    >>>> END_USE(vq);
    >>>> return -ENOSPC;
    >>>> }
    >>>>
    >>>>

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