Date: Tue, 25 Jun 2024 15:46:12 +0200
From: Stefano Garzarella <sgarzare@...hat.com>
To: Arseniy Krasnov <avkrasnov@...utedevices.com>
Cc: 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>, 
	"Michael S. Tsirkin" <mst@...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, Alexander Graf <graf@...zon.com>
Subject: Re: [RFC PATCH v1 1/2] virtio/vsock: rework deferred credit update
 logic

On Fri, Jun 21, 2024 at 10:25:40PM GMT, Arseniy Krasnov wrote:
>Previous calculation of 'free_space' was wrong (but worked as expected
>in most cases, see below), because it didn't account number of bytes in
>rx queue. Let's rework 'free_space' calculation in the following way:
>as this value is considered free space at rx side from tx point of view,
>it must be equal to return value of 'virtio_transport_get_credit()' at
>tx side. This function uses 'tx_cnt' counter and 'peer_fwd_cnt': first
>is number of transmitted bytes (without wrap), second is last 'fwd_cnt'
>value received from rx. So let's use same approach at rx side during
>'free_space' calculation: add 'rx_cnt' counter which is number of
>received bytes (also without wrap) and subtract 'last_fwd_cnt' from it.
>Now we have:
>1) 'rx_cnt' == 'tx_cnt' at both sides.
>2) 'last_fwd_cnt' == 'peer_fwd_cnt' - because first is last 'fwd_cnt'
>   sent to tx, while second is last 'fwd_cnt' received from rx.
>
>Now 'free_space' is handled correctly and also we don't need

mmm, I don't know if it was wrong before, maybe we could say it was less 
accurate.

That said, could we have the same problem now if we have a lot of 
producers and the virtqueue becomes full?

>'low_rx_bytes' flag - this was more like a hack.
>
>Previous calculation of 'free_space' worked (in 99% cases), because if
>we take a look on behaviour of both expressions (new and previous):
>
>'(rx_cnt - last_fwd_cnt)' and '(fwd_cnt - last_fwd_cnt)'
>
>Both of them always grows up, with almost same "speed": only difference
>is that 'rx_cnt' is incremented earlier during packet is received,
>while 'fwd_cnt' in incremented when packet is read by user. So if 'rx_cnt'
>grows "faster", then resulting 'free_space' become smaller also, so we
>send credit updates a little bit more, but:
>
>  * 'free_space' calculation based on 'rx_cnt' gives the same value,
>    which tx sees as free space at rx side, so original idea of

Ditto, what happen if the virtqueue is full?

>    'free_space' is now implemented as planned.
>  * Hack with 'low_rx_bytes' now is not needed.

Yeah, so this patch should also mitigate issue reported by Alex (added 
in CC), right?

If yes, please mention that problem and add a Reported-by giving credit 
to Alex.

>
>Also here is some performance comparison between both versions of
>'free_space' calculation:
>
> *------*----------*----------*
> |      | 'rx_cnt' | previous |
> *------*----------*----------*
> |H -> G|   8.42   |   7.82   |
> *------*----------*----------*
> |G -> H|   11.6   |   12.1   |
> *------*----------*----------*

How many seconds did you run it? How many repetitions? There's a little 
discrepancy anyway, but I can't tell if it's just noise.

>
>As benchmark 'vsock-iperf' with default arguments was used. There is no
>significant performance difference before and after this patch.
>
>Signed-off-by: Arseniy Krasnov <avkrasnov@...utedevices.com>
>---
> include/linux/virtio_vsock.h            | 1 +
> net/vmw_vsock/virtio_transport_common.c | 8 +++-----
> 2 files changed, 4 insertions(+), 5 deletions(-)

Thanks for working on this, I'll do more tests but the approach LGTM.

Thanks,
Stefano

>
>diff --git a/include/linux/virtio_vsock.h b/include/linux/virtio_vsock.h
>index c82089dee0c8..3579491c411e 100644
>--- a/include/linux/virtio_vsock.h
>+++ b/include/linux/virtio_vsock.h
>@@ -135,6 +135,7 @@ struct virtio_vsock_sock {
> 	u32 peer_buf_alloc;
>
> 	/* Protected by rx_lock */
>+	u32 rx_cnt;
> 	u32 fwd_cnt;
> 	u32 last_fwd_cnt;
> 	u32 rx_bytes;
>diff --git a/net/vmw_vsock/virtio_transport_common.c b/net/vmw_vsock/virtio_transport_common.c
>index 16ff976a86e3..1d4e2328e06e 100644
>--- a/net/vmw_vsock/virtio_transport_common.c
>+++ b/net/vmw_vsock/virtio_transport_common.c
>@@ -441,6 +441,7 @@ static bool virtio_transport_inc_rx_pkt(struct virtio_vsock_sock *vvs,
> 		return false;
>
> 	vvs->rx_bytes += len;
>+	vvs->rx_cnt += len;
> 	return true;
> }
>
>@@ -558,7 +559,6 @@ virtio_transport_stream_do_dequeue(struct vsock_sock *vsk,
> 	size_t bytes, total = 0;
> 	struct sk_buff *skb;
> 	u32 fwd_cnt_delta;
>-	bool low_rx_bytes;
> 	int err = -EFAULT;
> 	u32 free_space;
>
>@@ -603,9 +603,7 @@ virtio_transport_stream_do_dequeue(struct vsock_sock *vsk,
> 	}
>
> 	fwd_cnt_delta = vvs->fwd_cnt - vvs->last_fwd_cnt;
>-	free_space = vvs->buf_alloc - fwd_cnt_delta;
>-	low_rx_bytes = (vvs->rx_bytes <
>-			sock_rcvlowat(sk_vsock(vsk), 0, INT_MAX));
>+	free_space = vvs->buf_alloc - (vvs->rx_cnt - vvs->last_fwd_cnt);
>
> 	spin_unlock_bh(&vvs->rx_lock);
>
>@@ -619,7 +617,7 @@ virtio_transport_stream_do_dequeue(struct vsock_sock *vsk,
> 	 * number of bytes in rx queue is not enough to wake up reader.
> 	 */
> 	if (fwd_cnt_delta &&
>-	    (free_space < VIRTIO_VSOCK_MAX_PKT_BUF_SIZE || low_rx_bytes))
>+	    (free_space < VIRTIO_VSOCK_MAX_PKT_BUF_SIZE))
> 		virtio_transport_send_credit_update(vsk);
>
> 	return total;
>-- 
>2.25.1
>
>

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