Date:   Tue, 21 Mar 2023 09:28:54 +0100
From:   Stefano Garzarella <sgarzare@redhat.com>
To:     Arseniy Krasnov <avkrasnov@sberdevices.ru>
Cc:     Stefan Hajnoczi <stefanha@redhat.com>,
        "David S. Miller" <davem@davemloft.net>,
        Eric Dumazet <edumazet@google.com>,
        Jakub Kicinski <kuba@kernel.org>,
        Paolo Abeni <pabeni@redhat.com>,
        Bobby Eshleman <bobby.eshleman@bytedance.com>,
        kvm@vger.kernel.org, virtualization@lists.linux-foundation.org,
        netdev@vger.kernel.org, linux-kernel@vger.kernel.org,
        kernel@sberdevices.ru, oxffffaa@gmail.com
Subject: Re: [RFC PATCH v2] virtio/vsock: allocate multiple skbuffs on tx
Message-ID: <20230321082854.jluiqjyc4n5k2vza@sgarzare-redhat>
References: <ea5725eb-6cb5-cf15-2938-34e335a442fa@sberdevices.ru>
 <20230320142959.2wwf474fiyp3ex5z@sgarzare-redhat>
 <2be688af-89a6-d903-017b-dafee3e48c33@sberdevices.ru>
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On Mon, Mar 20, 2023 at 09:02:19PM +0300, Arseniy Krasnov wrote:
>
>
>On 20.03.2023 17:29, Stefano Garzarella wrote:
>> On Sun, Mar 19, 2023 at 09:46:10PM +0300, Arseniy Krasnov wrote:
>>> This adds small optimization for tx path: instead of allocating single
>>> skbuff on every call to transport, allocate multiple skbuff's until
>>> credit space allows, thus trying to send as much as possible data without
>>> return to af_vsock.c.
>>>
>>> Signed-off-by: Arseniy Krasnov <AVKrasnov@sberdevices.ru>
>>> ---
>>> Link to v1:
>>> https://lore.kernel.org/netdev/2c52aa26-8181-d37a-bccd-a86bd3cbc6e1@sberdevices.ru/
>>>
>>> Changelog:
>>> v1 -> v2:
>>> - If sent something, return number of bytes sent (even in
>>> ? case of error). Return error only if failed to sent first
>>> ? skbuff.
>>>
>>> net/vmw_vsock/virtio_transport_common.c | 53 ++++++++++++++++++-------
>>> 1 file changed, 39 insertions(+), 14 deletions(-)
>>>
>>> diff --git a/net/vmw_vsock/virtio_transport_common.c b/net/vmw_vsock/virtio_transport_common.c
>>> index 6564192e7f20..3fdf1433ec28 100644
>>> --- a/net/vmw_vsock/virtio_transport_common.c
>>> +++ b/net/vmw_vsock/virtio_transport_common.c
>>> @@ -196,7 +196,8 @@ static int virtio_transport_send_pkt_info(struct vsock_sock *vsk,
>>> ????const struct virtio_transport *t_ops;
>>> ????struct virtio_vsock_sock *vvs;
>>> ????u32 pkt_len = info->pkt_len;
>>> -??? struct sk_buff *skb;
>>> +??? u32 rest_len;
>>> +??? int ret;
>>>
>>> ????info->type = virtio_transport_get_type(sk_vsock(vsk));
>>>
>>> @@ -216,10 +217,6 @@ static int virtio_transport_send_pkt_info(struct vsock_sock *vsk,
>>>
>>> ????vvs = vsk->trans;
>>>
>>> -??? /* we can send less than pkt_len bytes */
>>> -??? if (pkt_len > VIRTIO_VSOCK_MAX_PKT_BUF_SIZE)
>>> -??????? pkt_len = VIRTIO_VSOCK_MAX_PKT_BUF_SIZE;
>>> -
>>> ????/* virtio_transport_get_credit might return less than pkt_len credit */
>>> ????pkt_len = virtio_transport_get_credit(vvs, pkt_len);
>>>
>>> @@ -227,17 +224,45 @@ static int virtio_transport_send_pkt_info(struct vsock_sock *vsk,
>>> ????if (pkt_len == 0 && info->op == VIRTIO_VSOCK_OP_RW)
>>> ??????? return pkt_len;
>>>
>>> -??? skb = virtio_transport_alloc_skb(info, pkt_len,
>>> -???????????????????? src_cid, src_port,
>>> -???????????????????? dst_cid, dst_port);
>>> -??? if (!skb) {
>>> -??????? virtio_transport_put_credit(vvs, pkt_len);
>>> -??????? return -ENOMEM;
>>> -??? }
>>> +??? ret = 0;
>>> +??? rest_len = pkt_len;
>>> +
>>> +??? do {
>>> +??????? struct sk_buff *skb;
>>> +??????? size_t skb_len;
>>> +
>>> +??????? skb_len = min_t(u32, VIRTIO_VSOCK_MAX_PKT_BUF_SIZE, rest_len);
>>> +
>>> +??????? skb = virtio_transport_alloc_skb(info, skb_len,
>>> +???????????????????????? src_cid, src_port,
>>> +???????????????????????? dst_cid, dst_port);
>>> +??????? if (!skb) {
>>> +??????????? ret = -ENOMEM;
>>> +??????????? break;
>>> +??????? }
>>> +
>>> +??????? virtio_transport_inc_tx_pkt(vvs, skb);
>>> +
>>> +??????? ret = t_ops->send_pkt(skb);
>>> +
>>> +??????? if (ret < 0)
>>> +??????????? break;
>>>
>>> -??? virtio_transport_inc_tx_pkt(vvs, skb);
>>> +??????? rest_len -= skb_len;
>>
>> t_ops->send_pkt() is returning the number of bytes sent. Current
>> implementations always return `skb_len`, so there should be no problem,
>> but it would be better to put a comment here, or we should handle the
>> case where ret != skb_len to avoid future issues.
>
>Hello, thanks for review!
>
>I see. I think i'll handle such partial sends (ret != skb_len) as error, as
>it is the only thing to do - we remove 'skb_len' from user's buffer, but
>'send_pkt()' returns another value, so it will be strange for me to continue
>this tx loop as everything is ok. Something like this:
>+
>+ if (ret < 0)
>+    break;
>+
>+ if (ret != skb_len) {
>+    ret = -EFAULT;//or may be -EIO
>+    break;
>+ }

Good for me.

>
>>
>>> +??? } while (rest_len);
>>>
>>> -??? return t_ops->send_pkt(skb);
>>> +??? /* Don't call this function with zero as argument:
>>> +???? * it tries to acquire spinlock and such argument
>>> +???? * makes this call useless.
>>
>> Good point, can we do the same also for virtio_transport_get_credit()?
>> (Maybe in a separate patch)
>>
>> I'm thinking if may be better to do it directly inside the functions,
>> but I don't have a strong opinion on that since we only call them here.
>>
>
>I think in this patch i can call 'virtio_transport_put_credit()' without if, but
>i'll prepare separate patch which adds zero argument check to this function.

Yep, I agree.

>As i see, the only function suitable for such 'if' condition is
>'virtio_transport_put_credit()'.

Why not even for virtio_transport_get_credit() ?

When we send packets without payload (e.g. VIRTIO_VSOCK_OP_REQUEST,
VIRTIO_VSOCK_OP_SHUTDOWN) we call virtio_transport_get_credit()
with `credit` parameter equal to 0, then we acquire the spinlock but
in the end we do nothing.

>Anyway - for future use this check won't be bad.

Yep, these are minor improvements ;-)

Thanks,
Stefano