Re: [PATCH net-next v7 3/4] virtio/vsock: fix logic which reduces credit update messages

[Stefano Garzarella]

On Thu, Dec 07, 2023 at 01:50:05AM +0300, Arseniy Krasnov wrote:
> On 07.12.2023 01:08, Michael S. Tsirkin wrote:
> > On Thu, Dec 07, 2023 at 12:52:51AM +0300, Arseniy Krasnov wrote:
> > > On 07.12.2023 00:53, Michael S. Tsirkin wrote:
> > > > On Thu, Dec 07, 2023 at 12:18:48AM +0300, Arseniy Krasnov wrote:
> > > > > Add one more condition for sending credit update during dequeue from
> > > > > stream socket: when number of bytes in the rx queue is smaller than
> > > > > SO_RCVLOWAT value of the socket. This is actual for non-default value
> > > > > of SO_RCVLOWAT (e.g. not 1) - idea is to "kick" peer to continue data
> > > > > transmission, because we need at least SO_RCVLOWAT bytes in our rx
> > > > > queue to wake up user for reading data (in corner case it is also
> > > > > possible to stuck both tx and rx sides, this is why 'Fixes' is used).
> > > > > Also handle case when 'fwd_cnt' wraps, while 'last_fwd_cnt' is still
> > > > > not.
> > > > >
> > > > > Fixes: b89d882dc9fc ("vsock/virtio: reduce credit update messages")
> > > > > Signed-off-by: Arseniy Krasnov <avkrasnov@xxxxxxxxxxxxxxxxx>
> > > > > ---
> > > > >  Changelog:
> > > > >  v6 -> v7:
> > > > >   * Handle wrap of 'fwd_cnt'.
> > > > >   * Do to send credit update when 'fwd_cnt' == 'last_fwd_cnt'.
> > > > >
> > > > >  net/vmw_vsock/virtio_transport_common.c | 18 +++++++++++++++---
> > > > >  1 file changed, 15 insertions(+), 3 deletions(-)
> > > > >
> > > > > diff --git a/net/vmw_vsock/virtio_transport_common.c b/net/vmw_vsock/virtio_transport_common.c
> > > > > index e137d740804e..39f8660d825d 100644
> > > > > --- a/net/vmw_vsock/virtio_transport_common.c
> > > > > +++ b/net/vmw_vsock/virtio_transport_common.c
> > > > > @@ -558,6 +558,8 @@ virtio_transport_stream_do_dequeue(struct vsock_sock *vsk,
> > > > >  	struct virtio_vsock_sock *vvs = vsk->trans;
> > > > >  	size_t bytes, total = 0;
> > > > >  	struct sk_buff *skb;
> > > > > +	u32 fwd_cnt_delta;
> > > > > +	bool low_rx_bytes;
> > > > >  	int err = -EFAULT;
> > > > >  	u32 free_space;
> > > > >
> > > > > @@ -601,7 +603,15 @@ virtio_transport_stream_do_dequeue(struct vsock_sock *vsk,
> > > > >  		}
> > > > >  	}
> > > > >
> > > > > -	free_space = vvs->buf_alloc - (vvs->fwd_cnt - vvs->last_fwd_cnt);
> > > > > +	/* Handle wrap of 'fwd_cnt'. */
> > > > > +	if (vvs->fwd_cnt < vvs->last_fwd_cnt)
> > > > > +		fwd_cnt_delta = vvs->fwd_cnt + (U32_MAX - vvs->last_fwd_cnt);
> > > >
> > > > Are you sure there's no off by one here? for example if fwd_cnt is 0
> > > > and last_fwd_cnt is 0xfffffffff then apparently delta is 0.
> > >
> > > Seems yes, I need +1 here
> >
> > And then you will get a nop, because assigning U32_MAX + 1 to u32
> > gives you 0. Adding () does nothing to change the result,
> > + and - are commutative.
>
> Ahh, unsigned here, yes.

Ooops, sorry I was confused here!

> @Stefano, what did You mean about wrapping here?
>
> I think Michael is right, for example

Yep, I agree!
Sorry for this wrong suggestion!

Stefano

> vvs->fwd_cnt wraps and now == 5
> vvs->last_fwd_cnt == 0xffffffff
>
> now delta before this patch will be 6 - correct value
>
> May be I didn't get your idea, so implement it very naive?
>
> Thanks, Arseniy
>
> > > > > +	else
> > > > > +		fwd_cnt_delta = vvs->fwd_cnt - vvs->last_fwd_cnt;
> > > >
> > > > I actually don't see what is wrong with just
> > > > 	fwd_cnt_delta = vvs->fwd_cnt - vvs->last_fwd_cnt
> > > > 32 bit unsigned math will I think handle wrap around correctly.
> > > >
> > > > And given buf_alloc is also u32 - I don't see where the bug is in
> > > > the original code.
> > >
> > > I think problem is when fwd_cnt wraps, while last_fwd_cnt is not. In this
> > > case fwd_cnt_delta will be too big, so we won't send credit update which
> > > leads to stall for sender
> > >
> > > Thanks, Arseniy
> >
> > Care coming up with an example?
> >
> >
> > > > > +
> > > > > +	free_space = vvs->buf_alloc - fwd_cnt_delta;
> > > > > +	low_rx_bytes = (vvs->rx_bytes <
> > > > > +			sock_rcvlowat(sk_vsock(vsk), 0, INT_MAX));
> > > > >
> > > > >  	spin_unlock_bh(&vvs->rx_lock);
> > > > >
> > > > > @@ -611,9 +621,11 @@ virtio_transport_stream_do_dequeue(struct vsock_sock *vsk,
> > > > >  	 * too high causes extra messages. Too low causes transmitter
> > > > >  	 * stalls. As stalls are in theory more expensive than extra
> > > > >  	 * messages, we set the limit to a high value. TODO: experiment
> > > > > -	 * with different values.
> > > > > +	 * with different values. Also send credit update message when
> > > > > +	 * number of bytes in rx queue is not enough to wake up reader.
> > > > >  	 */
> > > > > -	if (free_space < VIRTIO_VSOCK_MAX_PKT_BUF_SIZE)
> > > > > +	if (fwd_cnt_delta &&
> > > > > +	    (free_space < VIRTIO_VSOCK_MAX_PKT_BUF_SIZE || low_rx_bytes))
> > > > >  		virtio_transport_send_credit_update(vsk);
> > > > >
> > > > >  	return total;
> > > > > --
> > > > > 2.25.1