Messages in this thread | | | Subject | Re: [PATCH net-next v2 3/5] virtio_ring: add packed ring support | From | Jason Wang <> | Date | Fri, 9 Nov 2018 10:30:50 +0800 |
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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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