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 18:05:23 +0800 |
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On 2018/11/9 下午12:00, Michael S. Tsirkin wrote: > On Fri, Nov 09, 2018 at 10:30:50AM +0800, Jason Wang wrote: >> 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 > Yes we can but I doubt anyone does. > >> and the >> timer will finally fired and we can go. > on tx ring full we probably don't want to wait for timer. > But I think we can just prevent qemu from using tx timer > with virtio 1.
Yes, we can.
Thanks
> >>> 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 a quality of implementation issue, yes. > >>> 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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