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Message-ID: <smu77vmxw3ki36xhqnhtvujwswvkg5gkfwnt4vr5bnwljclseh@inbewbwkcqxs> Date: Fri, 1 Dec 2023 09:27:47 +0100 From: Stefano Garzarella <sgarzare@...hat.com> To: "Michael S. Tsirkin" <mst@...hat.com> Cc: Arseniy Krasnov <avkrasnov@...utedevices.com>, Stefan Hajnoczi <stefanha@...hat.com>, "David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, Jason Wang <jasowang@...hat.com>, Bobby Eshleman <bobby.eshleman@...edance.com>, kvm@...r.kernel.org, virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org, linux-kernel@...r.kernel.org, kernel@...rdevices.ru, oxffffaa@...il.com Subject: Re: [PATCH net-next v5 2/3] virtio/vsock: send credit update during setting SO_RCVLOWAT On Thu, Nov 30, 2023 at 12:40:43PM -0500, Michael S. Tsirkin wrote: >On Thu, Nov 30, 2023 at 03:11:19PM +0100, Stefano Garzarella wrote: >> On Thu, Nov 30, 2023 at 08:58:58AM -0500, Michael S. Tsirkin wrote: >> > On Thu, Nov 30, 2023 at 04:43:34PM +0300, Arseniy Krasnov wrote: >> > > >> > > >> > > On 30.11.2023 16:42, Michael S. Tsirkin wrote: >> > > > On Thu, Nov 30, 2023 at 04:08:39PM +0300, Arseniy Krasnov wrote: >> > > >> Send credit update message when SO_RCVLOWAT is updated and it is bigger >> > > >> than number of bytes in rx queue. It is needed, because 'poll()' will >> > > >> wait until number of bytes in rx queue will be not smaller than >> > > >> SO_RCVLOWAT, so kick sender to send more data. Otherwise mutual hungup >> > > >> for tx/rx is possible: sender waits for free space and receiver is >> > > >> waiting data in 'poll()'. >> > > >> >> > > >> Signed-off-by: Arseniy Krasnov <avkrasnov@...utedevices.com> >> > > >> --- >> > > >> Changelog: >> > > >> v1 -> v2: >> > > >> * Update commit message by removing 'This patch adds XXX' manner. >> > > >> * Do not initialize 'send_update' variable - set it directly during >> > > >> first usage. >> > > >> v3 -> v4: >> > > >> * Fit comment in 'virtio_transport_notify_set_rcvlowat()' to 80 chars. >> > > >> v4 -> v5: >> > > >> * Do not change callbacks order in transport structures. >> > > >> >> > > >> drivers/vhost/vsock.c | 1 + >> > > >> include/linux/virtio_vsock.h | 1 + >> > > >> net/vmw_vsock/virtio_transport.c | 1 + >> > > >> net/vmw_vsock/virtio_transport_common.c | 27 +++++++++++++++++++++++++ >> > > >> net/vmw_vsock/vsock_loopback.c | 1 + >> > > >> 5 files changed, 31 insertions(+) >> > > >> >> > > >> diff --git a/drivers/vhost/vsock.c b/drivers/vhost/vsock.c >> > > >> index f75731396b7e..4146f80db8ac 100644 >> > > >> --- a/drivers/vhost/vsock.c >> > > >> +++ b/drivers/vhost/vsock.c >> > > >> @@ -451,6 +451,7 @@ static struct virtio_transport vhost_transport = { >> > > >> .notify_buffer_size = virtio_transport_notify_buffer_size, >> > > >> >> > > >> .read_skb = virtio_transport_read_skb, >> > > >> + .notify_set_rcvlowat = virtio_transport_notify_set_rcvlowat >> > > >> }, >> > > >> >> > > >> .send_pkt = vhost_transport_send_pkt, >> > > >> diff --git a/include/linux/virtio_vsock.h b/include/linux/virtio_vsock.h >> > > >> index ebb3ce63d64d..c82089dee0c8 100644 >> > > >> --- a/include/linux/virtio_vsock.h >> > > >> +++ b/include/linux/virtio_vsock.h >> > > >> @@ -256,4 +256,5 @@ void virtio_transport_put_credit(struct virtio_vsock_sock *vvs, u32 credit); >> > > >> void virtio_transport_deliver_tap_pkt(struct sk_buff *skb); >> > > >> int virtio_transport_purge_skbs(void *vsk, struct sk_buff_head *list); >> > > >> int virtio_transport_read_skb(struct vsock_sock *vsk, skb_read_actor_t read_actor); >> > > >> +int virtio_transport_notify_set_rcvlowat(struct vsock_sock *vsk, int val); >> > > >> #endif /* _LINUX_VIRTIO_VSOCK_H */ >> > > >> diff --git a/net/vmw_vsock/virtio_transport.c b/net/vmw_vsock/virtio_transport.c >> > > >> index af5bab1acee1..8007593a3a93 100644 >> > > >> --- a/net/vmw_vsock/virtio_transport.c >> > > >> +++ b/net/vmw_vsock/virtio_transport.c >> > > >> @@ -539,6 +539,7 @@ static struct virtio_transport virtio_transport = { >> > > >> .notify_buffer_size = virtio_transport_notify_buffer_size, >> > > >> >> > > >> .read_skb = virtio_transport_read_skb, >> > > >> + .notify_set_rcvlowat = virtio_transport_notify_set_rcvlowat >> > > >> }, >> > > >> >> > > >> .send_pkt = virtio_transport_send_pkt, >> > > >> diff --git a/net/vmw_vsock/virtio_transport_common.c b/net/vmw_vsock/virtio_transport_common.c >> > > >> index f6dc896bf44c..1cb556ad4597 100644 >> > > >> --- a/net/vmw_vsock/virtio_transport_common.c >> > > >> +++ b/net/vmw_vsock/virtio_transport_common.c >> > > >> @@ -1684,6 +1684,33 @@ int virtio_transport_read_skb(struct vsock_sock *vsk, skb_read_actor_t recv_acto >> > > >> } >> > > >> EXPORT_SYMBOL_GPL(virtio_transport_read_skb); >> > > >> >> > > >> +int virtio_transport_notify_set_rcvlowat(struct vsock_sock *vsk, >> > > >> int val) >> > > >> +{ >> > > >> + struct virtio_vsock_sock *vvs = vsk->trans; >> > > >> + bool send_update; >> > > >> + >> > > >> + spin_lock_bh(&vvs->rx_lock); >> > > >> + >> > > >> + /* If number of available bytes is less than new SO_RCVLOWAT value, >> > > >> + * kick sender to send more data, because sender may sleep in >> > > >> its >> > > >> + * 'send()' syscall waiting for enough space at our side. >> > > >> + */ >> > > >> + send_update = vvs->rx_bytes < val; >> > > >> + >> > > >> + spin_unlock_bh(&vvs->rx_lock); >> > > >> + >> > > >> + if (send_update) { >> > > >> + int err; >> > > >> + >> > > >> + err = virtio_transport_send_credit_update(vsk); >> > > >> + if (err < 0) >> > > >> + return err; >> > > >> + } >> > > >> + >> > > >> + return 0; >> > > >> +} >> > > > >> > > > >> > > > I find it strange that this will send a credit update >> > > > even if nothing changed since this was called previously. >> > > > I'm not sure whether this is a problem protocol-wise, >> > > > but it certainly was not envisioned when the protocol was >> > > > built. WDYT? >> > > >> > > >From virtio spec I found: >> > > >> > > It is also valid to send a VIRTIO_VSOCK_OP_CREDIT_UPDATE packet without previously receiving a >> > > VIRTIO_VSOCK_OP_CREDIT_REQUEST packet. This allows communicating updates any time a change >> > > in buffer space occurs. >> > > So I guess there is no limitations to send such type of packet, e.g. it is not >> > > required to be a reply for some another packet. Please, correct me if im wrong. >> > > >> > > Thanks, Arseniy >> > >> > >> > Absolutely. My point was different - with this patch it is possible >> > that you are not adding any credits at all since the previous >> > VIRTIO_VSOCK_OP_CREDIT_UPDATE. >> >> I think the problem we're solving here is that since as an optimization we >> avoid sending the update for every byte we consume, but we put a threshold, >> then we make sure we update the peer. >> >> A credit update contains a snapshot and sending it the same as the previous >> one should not create any problem. > >Well it consumes a buffer on the other side. Sure, but we are already speculating by not updating the other side when we consume bytes before a certain threshold. This already avoids to consume many buffers. Here we're only sending it once, when the user sets RCVLOWAT, so basically I expect it won't affect performance. > >> My doubt now is that we only do this when we set RCVLOWAT , should we >> also >> do something when we consume bytes to avoid the optimization we have? >> >> Stefano > >Isn't this why we have credit request? Yep, but in practice we never use it. It would also consume 2 buffers, one at the transmitter and one at the receiver. However I agree that maybe we should start using it before we decide not to send any more data. To be compatible with older devices, though, I think for now we also need to send a credit update when the bytes in the receive queue are less than RCVLOWAT, as Arseniy proposed in the other series. Thanks, Stefano
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