Re: [PATCH v2 1/8] vsock/virtio: limit the memory used per-socket

[Stefano Garzarella <sgarzare@redhat.com>]

On Wed, May 15, 2019 at 10:48:44AM +0800, Jason Wang wrote:
> 
> 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:
> > > > > > +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.
> 

Jason, Stefan,
since I'm removing the patches to increase the buffers to 64 KiB and I'm
adding a threshold for small packets, I would simplify this patch,
removing the new buffer allocation and copying small packets into the
buffers already queued (if there is a space).
In this way, I should solve the issue of 1 byte packets.

Do you think could be better?

Thanks,
Stefano