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To: Eugenio Perez Martin <eperezma@redhat.com>,
Kevin Traynor <ktraynor@redhat.com>, Maxime Coquelin <mcoqueli@redhat.com>
Cc: dev@dpdk.org, "Liu, Yong" <yong.liu@intel.com>,
Adrian Moreno Zapata <amorenoz@redhat.com>, Jason Wang
<jasowang@redhat.com>, "Michael S. Tsirkin" <mst@redhat.com>
References: <bug-383-3@http.bugs.dpdk.org/>
<20200129193310.9157-1-eperezma@redhat.com>
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From: Maxime Coquelin <maxime.coquelin@redhat.com>
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Date: Tue, 4 Feb 2020 16:10:59 +0100
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Subject: Re: [dpdk-dev] [PATCH] vhost: flush shadow tx if there is no more
packets
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On 2/4/20 4:05 PM, Eugenio Perez Martin wrote:
> Ouch, my bad again, sorry :).
>=20
> I've forwarded the patch to stable@, please let me know if I need to do
> something else.
>=20
> Maxime, please let me know if I need to send a new version with the "Fixe=
d:
> " tag :).
Not needed, I will handle it.
Thanks for the fix,
Maxime
> Thanks!
>=20
> On Tue, Feb 4, 2020 at 2:49 PM Kevin Traynor <ktraynor@redhat.com> wrote:
>=20
>> 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@redhat.com>
>> wrote:
>>>
>>>> Hi Eugenio,
>>>>
>>>> On 29/01/2020 19:33, Eugenio P=C3=A9rez 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@dpdk.org
>>>>
>>>>> Signed-off-by: Eugenio P=C3=A9rez <eperezma@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 =3D vq->avail_wrap_counter;
>>>>> + uint16_t next_used_idx =3D vq->last_used_idx + 1;
>>>>> +
>>>>> + if (next_used_idx >=3D vq->size) {
>>>>> + next_used_idx -=3D vq->size;
>>>>> + wrap_counter ^=3D 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;
>>>>> }
>>>>>
>>>>>
>>>>
>>>>
>>>
>>
>>
>=20