[dpdk-dev] [PATCH] vhost: flush shadow tx if there is no more packets

[Maxime Coquelin]

On 2/4/20 4:05 PM, Eugenio Perez Martin wrote:
> Ouch, my bad again, sorry :).
> 
> I've forwarded the patch to stable@, please let me know if I need to do
> something else.
> 
> Maxime, please let me know if I need to send a new version with the "Fixed:
> " tag :).

Not needed, I will handle it.

Thanks for the fix,
Maxime

> Thanks!
> 
> On Tue, Feb 4, 2020 at 2:49 PM Kevin Traynor <ktraynor at redhat.com> wrote:
> 
>> On 04/02/2020 09:23, Eugenio Perez Martin wrote:
>>> Hi Kevin!
>>>
>>> Sorry, thanks for noticing it! It fixes commit ("31d6c6a5b vhost:
>> optimize
>>> packed ring dequeue"), what was not present on 18.11 version (I've
>> checked
>>> that v19.08 does not contain the failure).
>>>
>>
>> Right, in that case the issue is present on 19.11 stable, so it's worth
>> adding the tags to get it fixed in 19.11 stable.
>>
>>> Do I need to send another patch version with corrected commit message?
>>>
>>
>> Probably Maxime can do it on applying if you ask nicely :-)
>>
>>> Thanks!
>>>
>>> On Fri, Jan 31, 2020 at 7:38 PM Kevin Traynor <ktraynor at redhat.com>
>> wrote:
>>>
>>>> Hi Eugenio,
>>>>
>>>> On 29/01/2020 19:33, Eugenio Pérez wrote:
>>>>> The current implementation of vhost_net in packed vring tries to fill
>>>>> the shadow vector before send any actual changes to the guest. While
>>>>> this can be beneficial for the throughput, it conflicts with some
>>>>> bufferfloats methods like the linux kernel napi, that stops
>>>>> transmitting packets if there are too much bytes/buffers in the
>>>>> driver.
>>>>>
>>>>> To solve it, we flush the shadow packets at the end of
>>>>> virtio_dev_tx_packed if we have starved the vring, i.e., the next
>>>>> buffer is not available for the device.
>>>>>
>>>>> Since this last check can be expensive because of the atomic, we only
>>>>> check it if we have not obtained the expected (count) packets. If it
>>>>> happens to obtain "count" packets and there is no more available
>>>>> packets the caller needs to keep call virtio_dev_tx_packed again.
>>>>>
>>>>
>>>> It seems to be fixing an issue and should be considered for stable
>>>> branches? You can add the tags needed in the commit message here:
>>>>
>>>> Fixes: <commit that introduced bug/missed this case>
>>>> Cc: stable at dpdk.org
>>>>
>>>>> Signed-off-by: Eugenio Pérez <eperezma at redhat.com>
>>>>> ---
>>>>>  lib/librte_vhost/virtio_net.c | 27 ++++++++++++++++++++++++++-
>>>>>  1 file changed, 26 insertions(+), 1 deletion(-)
>>>>>
>>>>> diff --git a/lib/librte_vhost/virtio_net.c
>>>> b/lib/librte_vhost/virtio_net.c
>>>>> index 21c311732..ac2842b2d 100644
>>>>> --- a/lib/librte_vhost/virtio_net.c
>>>>> +++ b/lib/librte_vhost/virtio_net.c
>>>>> @@ -2133,6 +2133,20 @@ virtio_dev_tx_packed_zmbuf(struct virtio_net
>> *dev,
>>>>>       return pkt_idx;
>>>>>  }
>>>>>
>>>>> +static __rte_always_inline bool
>>>>> +next_desc_is_avail(const struct vhost_virtqueue *vq)
>>>>> +{
>>>>> +     bool wrap_counter = vq->avail_wrap_counter;
>>>>> +     uint16_t next_used_idx = vq->last_used_idx + 1;
>>>>> +
>>>>> +     if (next_used_idx >= vq->size) {
>>>>> +             next_used_idx -= vq->size;
>>>>> +             wrap_counter ^= 1;
>>>>> +     }
>>>>> +
>>>>> +     return desc_is_avail(&vq->desc_packed[next_used_idx],
>>>> wrap_counter);
>>>>> +}
>>>>> +
>>>>>  static __rte_noinline uint16_t
>>>>>  virtio_dev_tx_packed(struct virtio_net *dev,
>>>>>                    struct vhost_virtqueue *vq,
>>>>> @@ -2165,9 +2179,20 @@ virtio_dev_tx_packed(struct virtio_net *dev,
>>>>>
>>>>>       } while (remained);
>>>>>
>>>>> -     if (vq->shadow_used_idx)
>>>>> +     if (vq->shadow_used_idx) {
>>>>>               do_data_copy_dequeue(vq);
>>>>>
>>>>> +             if (remained && !next_desc_is_avail(vq)) {
>>>>> +                     /*
>>>>> +                      * The guest may be waiting to TX some buffers to
>>>>> +                      * enqueue more to avoid bufferfloat, so we try
>> to
>>>>> +                      * reduce latency here.
>>>>> +                      */
>>>>> +                     vhost_flush_dequeue_shadow_packed(dev, vq);
>>>>> +                     vhost_vring_call_packed(dev, vq);
>>>>> +             }
>>>>> +     }
>>>>> +
>>>>>       return pkt_idx;
>>>>>  }
>>>>>
>>>>>
>>>>
>>>>
>>>
>>
>>
>