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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: <20250912113432-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> <20250912112726-mutt-send-email-mst@kernel.org> <4418BA21-716E-468B-85EB-DB88CCD64F38@nutanix.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=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <4418BA21-716E-468B-85EB-DB88CCD64F38@nutanix.com> On Fri, Sep 12, 2025 at 03:33:32PM +0000, Jon Kohler wrote: > > > > On Sep 12, 2025, at 11:30 AM, Michael S. Tsirkin wrote: > > > > !-------------------------------------------------------------------| > > CAUTION: External Email > > > > |-------------------------------------------------------------------! > > > > 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. > > For the sake of discussion, let’s say vhost_tx_batch and the > sendmsg within took 1 full second to complete. A lot could potentially > happen in that amount of time. So sure, control path wise it looks like > we just checked it, but time wise, that could have been ages ago. Oh I forgot we had the tx batch in there. OK then, I don't have a problem with this. However, what I like about Jason's patch is that it is actually simply revert of your patch + a single call to vhost_tx_batch(net, nvq, sock, &msg); So it is a more obviosly correct approach. I'll be fine with doing what you propose on top, with testing that they are benefitial for performance. > > > > > >> 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. > > >