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List: kvm
Subject: Re: [PATCH net-next v5 2/3] virtio/vsock: send credit update during setting SO_RCVLOWAT
From: Arseniy Krasnov <avkrasnov () salutedevices ! com>
Date: 2023-11-30 17:49:09
Message-ID: 71d18598-1793-0c6c-7de6-f546befc47c1 () salutedevices ! com
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On 30.11.2023 20:37, Michael S. Tsirkin wrote:
> On Thu, Nov 30, 2023 at 06:41:56PM +0300, Arseniy Krasnov wrote:
> >
> >
> > On 30.11.2023 17:11, 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@salutedevices.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.
> > > 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?
> >
> > @Michael, Stefano just reproduced problem during bytes reading, but there is \
> > already old fix for this, which we forget to merge:) I think it must be included \
> > to this patchset.
> > https://lore.kernel.org/netdev/f304eabe-d2ef-11b1-f115-6967632f0339@sberdevices.ru/
> >
> > Thanks, Arseniy
>
>
> I generally don't merge patches tagged as RFC.
> Repost without that tag?
> Also, it looks like a bugfix we need either way, no?
I'll repost it without RFC as part of this patchset, also i'll add test for it
Thanks, Arseniy
>
> > >
> > > Stefano
> > >
>
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