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

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Date:   Tue, 14 May 2019 18:20:56 +0200
From:   Stefano Garzarella <sgarzare@...hat.com>
To:     Jason Wang <jasowang@...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 7/8] vsock/virtio: increase RX buffer size to 64 KiB

On Tue, May 14, 2019 at 11:38:05AM +0800, Jason Wang wrote:
> 
> On 2019/5/14 上午1:51, Stefano Garzarella wrote:
> > On Mon, May 13, 2019 at 06:01:52PM +0800, Jason Wang wrote:
> > > On 2019/5/10 下午8:58, 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@...hat.com>
> > > > ---
> > > >    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:
> 
>                 pkt->buf = kmalloc(buf_len, GFP_KERNEL);
>                 if (!pkt->buf) {
>                         virtio_transport_free_pkt(pkt);
>                         break;
>                 }
> 
> This is likely to fail when the memory is fragmented which is kind of
> fragile.
> 
> 

Thanks for pointing that out.

> > 
> > 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().
> 
> 

Yes, I agree.

> > 
> > 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.

I'll try to use kvec in the virtio_vsock_pkt.

Since this struct is shared also with the host driver (vhost-vsock),
I hope the changes could be limited, otherwise we can remove the last 2
patches of the series for now, leaving the RX buffer size to 4KB.

Thanks,
Stefano