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Tsirkin" To: Jason Wang Cc: eperezma@redhat.com, jonah.palmer@oracle.com, kuba@kernel.org, jon@nutanix.com, kvm@vger.kernel.org, virtualization@lists.linux.dev, netdev@vger.kernel.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org Subject: Re: [PATCH net V2 2/2] vhost-net: correctly flush batched packet before enabling notification Message-ID: <20250915073616-mutt-send-email-mst@kernel.org> References: <20250915024703.2206-1-jasowang@redhat.com> <20250915024703.2206-2-jasowang@redhat.com> Precedence: bulk X-Mailing-List: kvm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20250915024703.2206-2-jasowang@redhat.com> On Mon, Sep 15, 2025 at 10:47:03AM +0800, Jason Wang wrote: > Commit 8c2e6b26ffe2 ("vhost/net: Defer TX queue re-enable until after > sendmsg") tries to defer the notification enabling by moving the logic > out of the loop after the vhost_tx_batch() when nothing new is > spotted. This will bring side effects as the new logic would be reused > for several other error conditions. > > One example is the IOTLB: when there's an IOTLB miss, get_tx_bufs() > might return -EAGAIN and exit the loop and see there's still available > buffers, so it will queue the tx work again until userspace feed the > IOTLB entry correctly. This will slowdown the tx processing and > trigger the TX watchdog in the guest as reported in > https://lkml.org/lkml/2025/9/10/1596. > > Fixing this via partially reverting 8c2e6b26ffe2 and sticking the > notification enabling logic inside the loop when nothing new is > spotted and flush the batched before. > > Reported-by: Jon Kohler > Cc: stable@vger.kernel.org > Fixes: 8c2e6b26ffe2 ("vhost/net: Defer TX queue re-enable until after sendmsg") > Signed-off-by: Jason Wang I still think it's better to structure this as 2 patches: 1. the revert to fix the deadlock 2. one line addition of vhost_tx_batch(net, nvq, sock, &msg); to get back the performance of 8c2e6b26ffe2 Not critical though > --- > Changes since V1: > - Tweak the commit log > - Typo fixes but you didn't fix them all :( See below > --- > drivers/vhost/net.c | 33 +++++++++++++-------------------- > 1 file changed, 13 insertions(+), 20 deletions(-) > > diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c > index 16e39f3ab956..3611b7537932 100644 > --- a/drivers/vhost/net.c > +++ b/drivers/vhost/net.c > @@ -765,11 +765,11 @@ static void handle_tx_copy(struct vhost_net *net, struct socket *sock) > int err; > int sent_pkts = 0; > bool sock_can_batch = (sock->sk->sk_sndbuf == INT_MAX); > - bool busyloop_intr; > bool in_order = vhost_has_feature(vq, VIRTIO_F_IN_ORDER); > > do { > - busyloop_intr = false; > + bool busyloop_intr = false; > + > if (nvq->done_idx == VHOST_NET_BATCH) > vhost_tx_batch(net, nvq, sock, &msg); > > @@ -780,10 +780,18 @@ static void handle_tx_copy(struct vhost_net *net, struct socket *sock) > break; > /* Nothing new? Wait for eventfd to tell us they refilled. */ > if (head == vq->num) { > - /* Kicks are disabled at this point, break loop and > - * process any remaining batched packets. Queue will > - * be re-enabled afterwards. > + /* Flush batched packets before enabling > + * virqtueue notification to reduce notifications > + * unnecssary virtqueue kicks. unnecessary > */ > + vhost_tx_batch(net, nvq, sock, &msg); > + if (unlikely(busyloop_intr)) { > + vhost_poll_queue(&vq->poll); > + } else if (unlikely(vhost_enable_notify(&net->dev, > + vq))) { > + vhost_disable_notify(&net->dev, vq); > + continue; > + } > break; > } > > @@ -839,22 +847,7 @@ static void handle_tx_copy(struct vhost_net *net, struct socket *sock) > ++nvq->done_idx; > } while (likely(!vhost_exceeds_weight(vq, ++sent_pkts, total_len))); > > - /* Kicks are still disabled, dispatch any remaining batched msgs. */ > vhost_tx_batch(net, nvq, sock, &msg); > - > - if (unlikely(busyloop_intr)) > - /* If interrupted while doing busy polling, requeue the > - * handler to be fair handle_rx as well as other tasks > - * waiting on cpu. > - */ > - vhost_poll_queue(&vq->poll); > - else > - /* All of our work has been completed; however, before > - * leaving the TX handler, do one last check for work, > - * and requeue handler if necessary. If there is no work, > - * queue will be reenabled. > - */ > - vhost_net_busy_poll_try_queue(net, vq); > } > > static void handle_tx_zerocopy(struct vhost_net *net, struct socket *sock) > -- > 2.34.1