Date:   Wed, 15 May 2019 10:48:44 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Stefano Garzarella <sgarzare@...hat.com>
Cc:     netdev@...r.kernel.org, "David S. Miller" <davem@...emloft.net>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        virtualization@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Stefan Hajnoczi <stefanha@...hat.com>
Subject: Re: [PATCH v2 1/8] vsock/virtio: limit the memory used per-socket


On 2019/5/15 上午12:35, Stefano Garzarella wrote:
> On Tue, May 14, 2019 at 11:25:34AM +0800, Jason Wang wrote:
>> On 2019/5/14 上午1:23, Stefano Garzarella wrote:
>>> On Mon, May 13, 2019 at 05:58:53PM +0800, Jason Wang wrote:
>>>> On 2019/5/10 下午8:58, Stefano Garzarella wrote:
>>>>> Since virtio-vsock was introduced, the buffers filled by the host
>>>>> and pushed to the guest using the vring, are directly queued in
>>>>> a per-socket list avoiding to copy it.
>>>>> These buffers are preallocated by the guest with a fixed
>>>>> size (4 KB).
>>>>>
>>>>> The maximum amount of memory used by each socket should be
>>>>> controlled by the credit mechanism.
>>>>> The default credit available per-socket is 256 KB, but if we use
>>>>> only 1 byte per packet, the guest can queue up to 262144 of 4 KB
>>>>> buffers, using up to 1 GB of memory per-socket. In addition, the
>>>>> guest will continue to fill the vring with new 4 KB free buffers
>>>>> to avoid starvation of her sockets.
>>>>>
>>>>> This patch solves this issue copying the payload in a new buffer.
>>>>> Then it is queued in the per-socket list, and the 4KB buffer used
>>>>> by the host is freed.
>>>>>
>>>>> In this way, the memory used by each socket respects the credit
>>>>> available, and we still avoid starvation, paying the cost of an
>>>>> extra memory copy. When the buffer is completely full we do a
>>>>> "zero-copy", moving the buffer directly in the per-socket list.
>>>> I wonder in the long run we should use generic socket accouting mechanism
>>>> provided by kernel (e.g socket, skb, sndbuf, recvbug, truesize) instead of
>>>> vsock specific thing to avoid duplicating efforts.
>>> I agree, the idea is to switch to sk_buff but this should require an huge
>>> change. If we will use the virtio-net datapath, it will become simpler.
>>
>> Yes, unix domain socket is one example that uses general skb and socket
>> structure. And we probably need some kind of socket pair on host. Using
>> socket can also simplify the unification with vhost-net which depends on the
>> socket proto_ops to work. I admit it's a huge change probably, we can do it
>> gradually.
>>
> Yes, I also prefer to do this change gradually :)
>
>>>>> Signed-off-by: Stefano Garzarella <sgarzare@...hat.com>
>>>>> ---
>>>>>     drivers/vhost/vsock.c                   |  2 +
>>>>>     include/linux/virtio_vsock.h            |  8 +++
>>>>>     net/vmw_vsock/virtio_transport.c        |  1 +
>>>>>     net/vmw_vsock/virtio_transport_common.c | 95 ++++++++++++++++++-------
>>>>>     4 files changed, 81 insertions(+), 25 deletions(-)
>>>>>
>>>>> diff --git a/drivers/vhost/vsock.c b/drivers/vhost/vsock.c
>>>>> index bb5fc0e9fbc2..7964e2daee09 100644
>>>>> --- a/drivers/vhost/vsock.c
>>>>> +++ b/drivers/vhost/vsock.c
>>>>> @@ -320,6 +320,8 @@ vhost_vsock_alloc_pkt(struct vhost_virtqueue *vq,
>>>>>     		return NULL;
>>>>>     	}
>>>>> +	pkt->buf_len = pkt->len;
>>>>> +
>>>>>     	nbytes = copy_from_iter(pkt->buf, pkt->len, &iov_iter);
>>>>>     	if (nbytes != pkt->len) {
>>>>>     		vq_err(vq, "Expected %u byte payload, got %zu bytes\n",
>>>>> diff --git a/include/linux/virtio_vsock.h b/include/linux/virtio_vsock.h
>>>>> index e223e2632edd..345f04ee9193 100644
>>>>> --- a/include/linux/virtio_vsock.h
>>>>> +++ b/include/linux/virtio_vsock.h
>>>>> @@ -54,9 +54,17 @@ struct virtio_vsock_pkt {
>>>>>     	void *buf;
>>>>>     	u32 len;
>>>>>     	u32 off;
>>>>> +	u32 buf_len;
>>>>>     	bool reply;
>>>>>     };
>>>>> +struct virtio_vsock_buf {
>>>>> +	struct list_head list;
>>>>> +	void *addr;
>>>>> +	u32 len;
>>>>> +	u32 off;
>>>>> +};
>>>>> +
>>>>>     struct virtio_vsock_pkt_info {
>>>>>     	u32 remote_cid, remote_port;
>>>>>     	struct vsock_sock *vsk;
>>>>> diff --git a/net/vmw_vsock/virtio_transport.c b/net/vmw_vsock/virtio_transport.c
>>>>> index 15eb5d3d4750..af1d2ce12f54 100644
>>>>> --- a/net/vmw_vsock/virtio_transport.c
>>>>> +++ b/net/vmw_vsock/virtio_transport.c
>>>>> @@ -280,6 +280,7 @@ static void virtio_vsock_rx_fill(struct virtio_vsock *vsock)
>>>>>     			break;
>>>>>     		}
>>>>> +		pkt->buf_len = buf_len;
>>>>>     		pkt->len = buf_len;
>>>>>     		sg_init_one(&hdr, &pkt->hdr, sizeof(pkt->hdr));
>>>>> diff --git a/net/vmw_vsock/virtio_transport_common.c b/net/vmw_vsock/virtio_transport_common.c
>>>>> index 602715fc9a75..0248d6808755 100644
>>>>> --- a/net/vmw_vsock/virtio_transport_common.c
>>>>> +++ b/net/vmw_vsock/virtio_transport_common.c
>>>>> @@ -65,6 +65,9 @@ virtio_transport_alloc_pkt(struct virtio_vsock_pkt_info *info,
>>>>>     		pkt->buf = kmalloc(len, GFP_KERNEL);
>>>>>     		if (!pkt->buf)
>>>>>     			goto out_pkt;
>>>>> +
>>>>> +		pkt->buf_len = len;
>>>>> +
>>>>>     		err = memcpy_from_msg(pkt->buf, info->msg, len);
>>>>>     		if (err)
>>>>>     			goto out;
>>>>> @@ -86,6 +89,46 @@ virtio_transport_alloc_pkt(struct virtio_vsock_pkt_info *info,
>>>>>     	return NULL;
>>>>>     }
>>>>> +static struct virtio_vsock_buf *
>>>>> +virtio_transport_alloc_buf(struct virtio_vsock_pkt *pkt, bool zero_copy)
>>>>> +{
>>>>> +	struct virtio_vsock_buf *buf;
>>>>> +
>>>>> +	if (pkt->len == 0)
>>>>> +		return NULL;
>>>>> +
>>>>> +	buf = kzalloc(sizeof(*buf), GFP_KERNEL);
>>>>> +	if (!buf)
>>>>> +		return NULL;
>>>>> +
>>>>> +	/* If the buffer in the virtio_vsock_pkt is full, we can move it to
>>>>> +	 * the new virtio_vsock_buf avoiding the copy, because we are sure that
>>>>> +	 * we are not use more memory than that counted by the credit mechanism.
>>>>> +	 */
>>>>> +	if (zero_copy && pkt->len == pkt->buf_len) {
>>>>> +		buf->addr = pkt->buf;
>>>>> +		pkt->buf = NULL;
>>>>> +	} else {
>>>> Is the copy still needed if we're just few bytes less? We meet similar issue
>>>> for virito-net, and virtio-net solve this by always copy first 128bytes for
>>>> big packets.
>>>>
>>>> See receive_big()
>>> I'm seeing, It is more sophisticated.
>>> IIUC, virtio-net allocates a sk_buff with 128 bytes of buffer, then copies the
>>> first 128 bytes, then adds the buffer used to receive the packet as a frag to
>>> the skb.
>>
>> Yes and the point is if the packet is smaller than 128 bytes the pages will
>> be recycled.
>>
>>
> So it's avoid the overhead of allocation of a large buffer. I got it.
>
> Just a curiosity, why the threshold is 128 bytes?


 From its name (GOOD_COPY_LEN), I think it just a value that won't lose 
much performance, e.g the size two cachelines.

Thanks


>
>>> Do you suggest to implement something similar, or for now we can use my
>>> approach and if we will merge the datapath we can reuse the virtio-net
>>> approach?
>>
>> I think we need a better threshold. If I understand the patch correctly, we
>> will do copy unless the packet is 64K when guest is doing receiving. 1 byte
>> packet is indeed a problem, but we need to solve it without losing too much
>> performance.
> It is correct. I'll try to figure out a better threshold and the usage of
> order 0 page.
>
> Thanks again for your advices,
> Stefano

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