Re: [PATCH v2 7/8] vsock/virtio: increase RX buffer size to 64 KiB

[Jason Wang]

On 2019/5/14 äå1:51, Stefano Garzarella wrote:
> On Mon, May 13, 2019 at 06:01:52PM +0800, Jason Wang wrote:
> > On 2019/5/10 äå8:58, Stefano Garzarella wrote:
> > > In order to increase host -> guest throughput with large packets,
> > > we can use 64 KiB RX buffers.
> > >
> > > Signed-off-by: Stefano Garzarella <sgarzare@xxxxxxxxxx>
> > > ---
> > >    include/linux/virtio_vsock.h | 2 +-
> > >    1 file changed, 1 insertion(+), 1 deletion(-)
> > >
> > > diff --git a/include/linux/virtio_vsock.h b/include/linux/virtio_vsock.h
> > > index 84b72026d327..5a9d25be72df 100644
> > > --- a/include/linux/virtio_vsock.h
> > > +++ b/include/linux/virtio_vsock.h
> > > @@ -10,7 +10,7 @@
> > >    #define VIRTIO_VSOCK_DEFAULT_MIN_BUF_SIZE	128
> > >    #define VIRTIO_VSOCK_DEFAULT_BUF_SIZE		(1024 * 256)
> > >    #define VIRTIO_VSOCK_DEFAULT_MAX_BUF_SIZE	(1024 * 256)
> > > -#define VIRTIO_VSOCK_DEFAULT_RX_BUF_SIZE	(1024 * 4)
> > > +#define VIRTIO_VSOCK_DEFAULT_RX_BUF_SIZE	(1024 * 64)
> > >    #define VIRTIO_VSOCK_MAX_BUF_SIZE		0xFFFFFFFFUL
> > >    #define VIRTIO_VSOCK_MAX_PKT_BUF_SIZE		(1024 * 64)
> >
> > We probably don't want such high order allocation. It's better to switch to
> > use order 0 pages in this case. See add_recvbuf_big() for virtio-net. If we
> > get datapath unified, we will get more stuffs set.
> IIUC, you are suggesting to allocate only pages and put them in a
> scatterlist, then add them to the virtqueue.
>
> Is it correct?


Yes since you are using:

ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ pkt->buf = kmalloc(buf_len, GFP_KERNEL);
ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ if (!pkt->buf) {
ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ virtio_transport_free_pkt(pkt);
ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ break;
ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ }

This is likely to fail when the memory is fragmented which is kind of 
fragile.


> The issue that I have here, is that the virtio-vsock guest driver, see
> virtio_vsock_rx_fill(), allocates a struct virtio_vsock_pkt that
> contains the room for the header, then allocates the buffer for the payload.
> At this point it fills the scatterlist with the &virtio_vsock_pkt.hdr and the
> buffer for the payload.


This part should be fine since what is needed is just adding more pages 
to sg[] and call virtuqeueu_add_sg().


> Changing this will require several modifications, and if we get datapath
> unified, I'm not sure it's worth it.
> Of course, if we leave the datapaths separated, I'd like to do that later.
>
> What do you think?


For the driver it self, it should not be hard. But I think you mean the 
issue of e.g virtio_vsock_pkt itself which doesn't support sg. For short 
time, maybe we can use kvec instead.

Thanks


> Thanks,
> Stefano