Re: [PATCH v4 1/5] vsock/virtio: limit the memory used per-socket

[Stefan Hajnoczi <stefanha@gmail.com>]

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On Sun, Sep 01, 2019 at 02:56:44AM -0400, Michael S. Tsirkin wrote:
> On Fri, Aug 30, 2019 at 11:40:59AM +0200, Stefano Garzarella wrote:
> > On Mon, Jul 29, 2019 at 10:04:29AM -0400, Michael S. Tsirkin wrote:
> > > On Wed, Jul 17, 2019 at 01:30:26PM +0200, 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. 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 other sockets.
> > > > 
> > > > This patch mitigates this issue copying the payload of small
> > > > packets (< 128 bytes) into the buffer of last packet queued, in
> > > > order to avoid wasting memory.
> > > > 
> > > > Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
> > > > Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
> > > 
> > > This is good enough for net-next, but for net I think we
> > > should figure out how to address the issue completely.
> > > Can we make the accounting precise? What happens to
> > > performance if we do?
> > > 
> > 
> > Since I'm back from holidays, I'm restarting this thread to figure out
> > how to address the issue completely.
> > 
> > I did a better analysis of the credit mechanism that we implemented in
> > virtio-vsock to get a clearer view and I'd share it with you:
> > 
> >     This issue affect only the "host->guest" path. In this case, when the
> >     host wants to send a packet to the guest, it uses a "free" buffer
> >     allocated by the guest (4KB).
> >     The "free" buffers available for the host are shared between all
> >     sockets, instead, the credit mechanism is per-socket, I think to
> >     avoid the starvation of others sockets.
> >     The guests re-fill the "free" queue when the available buffers are
> >     less than half.
> > 
> >     Each peer have these variables in the per-socket state:
> >        /* local vars */
> >        buf_alloc        /* max bytes usable by this socket
> >                            [exposed to the other peer] */
> >        fwd_cnt          /* increased when RX packet is consumed by the
> >                            user space [exposed to the other peer] */
> >        tx_cnt 	        /* increased when TX packet is sent to the other peer */
> > 
> >        /* remote vars  */
> >        peer_buf_alloc   /* peer's buf_alloc */
> >        peer_fwd_cnt     /* peer's fwd_cnt */
> > 
> >     When a peer sends a packet, it increases the 'tx_cnt'; when the
> >     receiver consumes the packet (copy it to the user-space buffer), it
> >     increases the 'fwd_cnt'.
> >     Note: increments are made considering the payload length and not the
> >     buffer length.
> > 
> >     The value of 'buf_alloc' and 'fwd_cnt' are sent to the other peer in
> >     all packet headers or with an explicit CREDIT_UPDATE packet.
> > 
> >     The local 'buf_alloc' value can be modified by the user space using
> >     setsockopt() with optname=SO_VM_SOCKETS_BUFFER_SIZE.
> > 
> >     Before to send a packet, the peer checks the space available:
> >     	credit_available = peer_buf_alloc - (tx_cnt - peer_fwd_cnt)
> >     and it will send up to credit_available bytes to the other peer.
> > 
> > Possible solutions considering Michael's advice:
> > 1. Use the buffer length instead of the payload length when we increment
> >    the counters:
> >   - This approach will account precisely the memory used per socket.
> >   - This requires changes in both guest and host.
> >   - It is not compatible with old drivers, so a feature should be negotiated.
> > 2. Decrease the advertised 'buf_alloc' taking count of bytes queued in
> >    the socket queue but not used. (e.g. 256 byte used on 4K available in
> >    the buffer)
> >   - pkt->hdr.buf_alloc = buf_alloc - bytes_not_used.
> >   - This should be compatible also with old drivers.
> > 
> > Maybe the second is less invasive, but will it be too tricky?
> > Any other advice or suggestions?
> > 
> > Thanks in advance,
> > Stefano
> 
> OK let me try to clarify.  The idea is this:
> 
> Let's say we queue a buffer of 4K, and we copy if len < 128 bytes.  This
> means that in the worst case (128 byte packets), each byte of credit in
> the socket uses up 4K/128 = 16 bytes of kernel memory. In fact we need
> to also account for the virtio_vsock_pkt since I think it's kept around
> until userspace consumes it.
> 
> Thus given X buf alloc allowed in the socket, we should publish X/16
> credits to the other side. This will ensure the other side does not send
> more than X/16 bytes for a given socket and thus we won't need to
> allocate more than X bytes to hold the data.
> 
> We can play with the copy break value to tweak this.

This seems like a reasonable solution.  Hopefully the benchmark results
will come out okay too.

Stefan

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