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@xxxxxxxxxx> > > > Signed-off-by: Stefano Garzarella <sgarzare@xxxxxxxxxx> > > > > 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.
