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Tsirkin" To: Arseniy Krasnov Cc: Stefan Hajnoczi , Stefano Garzarella , "David S. Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , Jason Wang , Bobby Eshleman , kvm@vger.kernel.org, virtualization@lists.linux-foundation.org, netdev@vger.kernel.org, linux-kernel@vger.kernel.org, kernel@sberdevices.ru, oxffffaa@gmail.com Subject: Re: [PATCH net-next v7 3/4] virtio/vsock: fix logic which reduces credit update messages Message-ID: <20231206170640-mutt-send-email-mst@kernel.org> References: <20231206211849.2707151-1-avkrasnov@salutedevices.com> <20231206211849.2707151-4-avkrasnov@salutedevices.com> <20231206165045-mutt-send-email-mst@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 > >> --- > >> 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. > > > > > >> + 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 > >