Re: [RFC PATCH v2] virtio/vsock: allocate multiple skbuffs on tx

[Stefano Garzarella <sgarzare@redhat.com>]

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