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[80.230.39.63]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-479052def4bsm26279025e9.13.2025.11.25.15.50.22 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 25 Nov 2025 15:50:23 -0800 (PST) Date: Tue, 25 Nov 2025 18:50:20 -0500 From: "Michael S. Tsirkin" To: Jon Kohler Cc: Jason Wang , Eugenio =?iso-8859-1?Q?P=E9rez?= , kvm@vger.kernel.org, virtualization@lists.linux.dev, netdev@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH net-next] vhost/net: check peek_head_len after signal to guest to avoid delays Message-ID: <20251125184936-mutt-send-email-mst@kernel.org> References: <20251125180034.1167847-1-jon@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=us-ascii Content-Disposition: inline In-Reply-To: <20251125180034.1167847-1-jon@nutanix.com> On Tue, Nov 25, 2025 at 11:00:33AM -0700, Jon Kohler wrote: > In non-busypoll handle_rx paths, if peek_head_len returns 0, the RX > loop breaks, the RX wait queue is re-enabled, and vhost_net_signal_used > is called to flush done_idx and notify the guest if needed. > > However, signaling the guest can take non-trivial time. During this > window, additional RX payloads may arrive on rx_ring without further > kicks. These new payloads will sit unprocessed until another kick > arrives, increasing latency. In high-rate UDP RX workloads, this was > observed to occur over 20k times per second. > > To minimize this window and improve opportunities to process packets > promptly, immediately call peek_head_len after signaling. If new packets > are found, treat it as a busy poll interrupt and requeue handle_rx, > improving fairness to TX handlers and other pending CPU work. This also > helps suppress unnecessary thread wakeups, reducing waker CPU demand. > > Signed-off-by: Jon Kohler Given this is supposed to be a performance improvement, pls include info on the effect this has on performance. Thanks! > --- > drivers/vhost/net.c | 21 +++++++++++++++++++++ > 1 file changed, 21 insertions(+) > > diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c > index 35ded4330431..04cb5f1dc6e4 100644 > --- a/drivers/vhost/net.c > +++ b/drivers/vhost/net.c > @@ -1015,6 +1015,27 @@ static int vhost_net_rx_peek_head_len(struct vhost_net *net, struct sock *sk, > struct vhost_virtqueue *tvq = &tnvq->vq; > int len = peek_head_len(rnvq, sk); > > + if (!len && rnvq->done_idx) { > + /* When idle, flush signal first, which can take some > + * time for ring management and guest notification. > + * Afterwards, check one last time for work, as the ring > + * may have received new work during the notification > + * window. > + */ > + vhost_net_signal_used(rnvq, *count); > + *count = 0; > + if (peek_head_len(rnvq, sk)) { > + /* More work came in during the notification > + * window. To be fair to the TX handler and other > + * potentially pending work items, pretend like > + * this was a busy poll interruption so that > + * the RX handler will be rescheduled and try > + * again. > + */ > + *busyloop_intr = true; > + } > + } > + > if (!len && rvq->busyloop_timeout) { > /* Flush batched heads first */ > vhost_net_signal_used(rnvq, *count); > -- > 2.43.0