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Tsirkin" To: Jason Wang Cc: andrew+netdev@lunn.ch, davem@davemloft.net, edumazet@google.com, pabeni@redhat.com, xuanzhuo@linux.alibaba.com, eperezma@redhat.com, virtualization@lists.linux.dev, netdev@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH net-next] virtio-net: tweak for better TX performance in NAPI mode Message-ID: <20250220162359-mutt-send-email-mst@kernel.org> References: <20250218023908.1755-1-jasowang@redhat.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=us-ascii Content-Disposition: inline In-Reply-To: <20250218023908.1755-1-jasowang@redhat.com> On Tue, Feb 18, 2025 at 10:39:08AM +0800, Jason Wang wrote: > There are several issues existed in start_xmit(): > > - Transmitted packets need to be freed before sending a packet, this > introduces delay and increases the average packets transmit > time. This also increase the time that spent in holding the TX lock. > - Notification is enabled after free_old_xmit_skbs() which will > introduce unnecessary interrupts if TX notification happens on the > same CPU that is doing the transmission now (actually, virtio-net > driver are optimized for this case). > > So this patch tries to avoid those issues by not cleaning transmitted > packets in start_xmit() when TX NAPI is enabled and disable > notifications even more aggressively. Notification will be since the > beginning of the start_xmit(). But we can't enable delayed > notification after TX is stopped as we will lose the > notifications. Instead, the delayed notification needs is enabled > after the virtqueue is kicked for best performance. > > Performance numbers: > > 1) single queue 2 vcpus guest with pktgen_sample03_burst_single_flow.sh > (burst 256) + testpmd (rxonly) on the host: > > - When pinning TX IRQ to pktgen VCPU: split virtqueue PPS were > increased 55% from 6.89 Mpps to 10.7 Mpps and 32% TX interrupts were > eliminated. Packed virtqueue PPS were increased 50% from 7.09 Mpps to > 10.7 Mpps, 99% TX interrupts were eliminated. > > - When pinning TX IRQ to VCPU other than pktgen: split virtqueue PPS > were increased 96% from 5.29 Mpps to 10.4 Mpps and 45% TX interrupts > were eliminated; Packed virtqueue PPS were increased 78% from 6.12 > Mpps to 10.9 Mpps and 99% TX interrupts were eliminated. > > 2) single queue 1 vcpu guest + vhost-net/TAP on the host: single > session netperf from guest to host shows 82% improvement from > 31Gb/s to 58Gb/s, %stddev were reduced from 34.5% to 1.9% and 88% > of TX interrupts were eliminated. > > Signed-off-by: Jason Wang okay Acked-by: Michael S. Tsirkin but tell me something, would it be even better to schedule napi once, and have that deal with enabling notifications? > --- > drivers/net/virtio_net.c | 45 ++++++++++++++++++++++++++++------------ > 1 file changed, 32 insertions(+), 13 deletions(-) > > diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c > index 7646ddd9bef7..ac26a6201c44 100644 > --- a/drivers/net/virtio_net.c > +++ b/drivers/net/virtio_net.c > @@ -1088,11 +1088,10 @@ static bool is_xdp_raw_buffer_queue(struct virtnet_info *vi, int q) > return false; > } > > -static void check_sq_full_and_disable(struct virtnet_info *vi, > - struct net_device *dev, > - struct send_queue *sq) > +static bool tx_may_stop(struct virtnet_info *vi, > + struct net_device *dev, > + struct send_queue *sq) > { > - bool use_napi = sq->napi.weight; > int qnum; > > qnum = sq - vi->sq; > @@ -1114,6 +1113,25 @@ static void check_sq_full_and_disable(struct virtnet_info *vi, > u64_stats_update_begin(&sq->stats.syncp); > u64_stats_inc(&sq->stats.stop); > u64_stats_update_end(&sq->stats.syncp); > + > + return true; > + } > + > + return false; > +} > + > +static void check_sq_full_and_disable(struct virtnet_info *vi, > + struct net_device *dev, > + struct send_queue *sq) > +{ > + bool use_napi = sq->napi.weight; > + int qnum; > + > + qnum = sq - vi->sq; > + > + if (tx_may_stop(vi, dev, sq)) { > + struct netdev_queue *txq = netdev_get_tx_queue(dev, qnum); > + > if (use_napi) { > if (unlikely(!virtqueue_enable_cb_delayed(sq->vq))) > virtqueue_napi_schedule(&sq->napi, sq->vq); > @@ -3253,15 +3271,10 @@ static netdev_tx_t start_xmit(struct sk_buff *skb, struct net_device *dev) > bool use_napi = sq->napi.weight; > bool kick; > > - /* Free up any pending old buffers before queueing new ones. */ > - do { > - if (use_napi) > - virtqueue_disable_cb(sq->vq); > - > + if (!use_napi) > free_old_xmit(sq, txq, false); > - > - } while (use_napi && !xmit_more && > - unlikely(!virtqueue_enable_cb_delayed(sq->vq))); > + else > + virtqueue_disable_cb(sq->vq); > > /* timestamp packet in software */ > skb_tx_timestamp(skb); > @@ -3287,7 +3300,10 @@ static netdev_tx_t start_xmit(struct sk_buff *skb, struct net_device *dev) > nf_reset_ct(skb); > } > > - check_sq_full_and_disable(vi, dev, sq); > + if (use_napi) > + tx_may_stop(vi, dev, sq); > + else > + check_sq_full_and_disable(vi, dev,sq); > > kick = use_napi ? __netdev_tx_sent_queue(txq, skb->len, xmit_more) : > !xmit_more || netif_xmit_stopped(txq); > @@ -3299,6 +3315,9 @@ static netdev_tx_t start_xmit(struct sk_buff *skb, struct net_device *dev) > } > } > > + if (use_napi && kick && unlikely(!virtqueue_enable_cb_delayed(sq->vq))) > + virtqueue_napi_schedule(&sq->napi, sq->vq); > + > return NETDEV_TX_OK; > } > > -- > 2.34.1