Re: [RFC v3 3/7] vsock: batch buffers in tx

[Guo Zhi <qtxuning1999@sjtu.edu.cn>]

----- Original Message -----
> From: "jasowang" <jasowang@redhat.com>
> To: "Guo Zhi" <qtxuning1999@sjtu.edu.cn>, "eperezma" <eperezma@redhat.com>, "sgarzare" <sgarzare@redhat.com>, "Michael
> Tsirkin" <mst@redhat.com>
> Cc: "netdev" <netdev@vger.kernel.org>, "linux-kernel" <linux-kernel@vger.kernel.org>, "kvm list" <kvm@vger.kernel.org>,
> "virtualization" <virtualization@lists.linux-foundation.org>
> Sent: Wednesday, September 7, 2022 12:27:40 PM
> Subject: Re: [RFC v3 3/7] vsock: batch buffers in tx

> 在 2022/9/1 13:54, Guo Zhi 写道:
>> Vsock uses buffers in order, and for tx driver doesn't have to
>> know the length of the buffer. So we can do a batch for vsock if
>> in order negotiated, only write one used ring for a batch of buffers
>>
>> Signed-off-by: Guo Zhi <qtxuning1999@sjtu.edu.cn>
>> ---
>>   drivers/vhost/vsock.c | 12 ++++++++++--
>>   1 file changed, 10 insertions(+), 2 deletions(-)
>>
>> diff --git a/drivers/vhost/vsock.c b/drivers/vhost/vsock.c
>> index 368330417bde..e08fbbb5439e 100644
>> --- a/drivers/vhost/vsock.c
>> +++ b/drivers/vhost/vsock.c
>> @@ -497,7 +497,7 @@ static void vhost_vsock_handle_tx_kick(struct vhost_work
>> *work)
>>   	struct vhost_vsock *vsock = container_of(vq->dev, struct vhost_vsock,
>>   						 dev);
>>   	struct virtio_vsock_pkt *pkt;
>> -	int head, pkts = 0, total_len = 0;
>> +	int head, pkts = 0, total_len = 0, add = 0;
>>   	unsigned int out, in;
>>   	bool added = false;
>>   
>> @@ -551,10 +551,18 @@ static void vhost_vsock_handle_tx_kick(struct vhost_work
>> *work)
>>   		else
>>   			virtio_transport_free_pkt(pkt);
>>   
>> -		vhost_add_used(vq, head, 0);
>> +		if (!vhost_has_feature(vq, VIRTIO_F_IN_ORDER)) {
>> +			vhost_add_used(vq, head, 0);
> 
> 
> I'd do this step by step.
> 
> 1) switch to use vhost_add_used_n() for vsock, less copy_to_user()
> better performance
> 2) do in-order on top
> 
> 
LGTM!, I think it is the correct way.

>> +		} else {
>> +			vq->heads[add].id = head;
>> +			vq->heads[add++].len = 0;
> 
> 
> How can we guarantee that we are in the boundary of the heads array?
> 
> Btw in the case of in-order we don't need to record the heads, instead
> we just need to know the head of the last buffer, it reduces the stress
> on the cache.
> 
> Thanks
> 
Yeah, I will change this and only copy last head for in order feature.

Thanks
> 
>> +		}
>>   		added = true;
>>   	} while(likely(!vhost_exceeds_weight(vq, ++pkts, total_len)));
>>   
>> +	/* If in order feature negotiaged, we can do a batch to increase performance
>> */
>> +	if (vhost_has_feature(vq, VIRTIO_F_IN_ORDER) && added)
>> +		vhost_add_used_n(vq, vq->heads, add);
>>   no_more_replies:
>>   	if (added)
>>   		vhost_signal(&vsock->dev, vq);