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@xxxxxxxxxxxxxxxxx>
---
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
