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Tsirkin" To: Jon Kohler Cc: Jason Wang , "eperezma@redhat.com" , "jonah.palmer@oracle.com" , "kuba@kernel.org" , "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 2/2] vhost-net: correctly flush batched packet before enabling notification Message-ID: <20250912112726-mutt-send-email-mst@kernel.org> References: <20250912082658.2262-1-jasowang@redhat.com> <20250912082658.2262-2-jasowang@redhat.com> <20250912044523-mutt-send-email-mst@kernel.org> <63426904-881F-4725-96F5-3343389ED170@nutanix.com> Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <63426904-881F-4725-96F5-3343389ED170@nutanix.com> On Fri, Sep 12, 2025 at 03:24:42PM +0000, Jon Kohler wrote: > > > > On Sep 12, 2025, at 4:50 AM, Michael S. Tsirkin wrote: > > > > !-------------------------------------------------------------------| > > CAUTION: External Email > > > > |-------------------------------------------------------------------! > > > > On Fri, Sep 12, 2025 at 04:26:58PM +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 may > >> trigger the TX watchdog in the guest. > > > > It's not that it might. > > Pls clarify that it *has been reported* to do exactly that, > > and add a link to the report. > > > > > >> Fixing this by stick the notificaiton 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 > > > > So this is mostly a revert, but with > > vhost_tx_batch(net, nvq, sock, &msg); > > added in to avoid regressing performance. > > > > If you do not want to structure it like this (revert+optimization), > > then pls make that clear in the message. > > > > > >> --- > >> 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 > >> + * unnecssary virtqueue kicks. > > > > typos: virtqueue, 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; > >> } > > See my comment below, but how about something like this? > if (head == vq->num) { > /* Flush batched packets before enabling > * virtqueue notification to reduce > * unnecessary virtqueue kicks. > */ > 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); I mean it's functionally equivalent, but vhost_net_busy_poll_try_queue checks the avail ring again and we just checked it. Why is this a good idea? This happens on good path so I dislike unnecessary work like this. > 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); > > Note: the use of vhost_net_busy_poll_try_queue was intentional in my > patch as it was checking to see both conditionals. > > Can we simply hoist my logic up instead? > > >> } > >> > >> static void handle_tx_zerocopy(struct vhost_net *net, struct socket *sock) > >> -- > >> 2.34.1 > > > > Tested-by: Jon Kohler > > > Tried this out on a 6.16 host / guest that locked up with iotlb miss loop, > applied this patch and all was well.