From mboxrd@z Thu Jan 1 00:00:00 1970 From: Rusty Russell Subject: Re: [PATCHv2 10/14] virtio_net: limit xmit polling Date: Sat, 21 May 2011 11:49:59 +0930 Message-ID: <877h9kvlps.fsf@rustcorp.com.au> References: Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Cc: Krishna Kumar , Carsten Otte , lguest-uLR06cmDAlY/bJ5BZ2RsiQ@public.gmane.org, Shirley Ma , kvm-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, linux-s390-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, netdev-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, habanero-23VcF4HTsmIX0ybBhKVfKdBPR1lH4CV8@public.gmane.org, Heiko Carstens , linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, virtualization-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org, steved-r/Jw6+rmf7HQT0dZR+AlfA@public.gmane.org, Christian Borntraeger , Tom Lendacky , Martin Schwidefsky , linux390-tA70FqPdS9bQT0dZR+AlfA@public.gmane.org To: "Michael S. Tsirkin" , linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org Return-path: In-Reply-To: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: lguest-bounces+glkvl-lguest=m.gmane.org-uLR06cmDAlY/bJ5BZ2RsiQ@public.gmane.org Sender: lguest-bounces+glkvl-lguest=m.gmane.org-uLR06cmDAlY/bJ5BZ2RsiQ@public.gmane.org List-Id: kvm.vger.kernel.org On Fri, 20 May 2011 02:11:56 +0300, "Michael S. Tsirkin" wrote: > Current code might introduce a lot of latency variation > if there are many pending bufs at the time we > attempt to transmit a new one. This is bad for > real-time applications and can't be good for TCP either. Do we have more than speculation to back that up, BTW? This patch is pretty sloppy; the previous ones were better polished. > -static void free_old_xmit_skbs(struct virtnet_info *vi) > +static bool free_old_xmit_skbs(struct virtnet_info *vi, int capacity) > { A comment here indicating it returns true if it frees something? > struct sk_buff *skb; > unsigned int len; > - > - while ((skb = virtqueue_get_buf(vi->svq, &len)) != NULL) { > + bool c; > + int n; > + > + /* We try to free up at least 2 skbs per one sent, so that we'll get > + * all of the memory back if they are used fast enough. */ > + for (n = 0; > + ((c = virtqueue_get_capacity(vi->svq) < capacity) || n < 2) && > + ((skb = virtqueue_get_buf(vi->svq, &len))); > + ++n) { > pr_debug("Sent skb %p\n", skb); > vi->dev->stats.tx_bytes += skb->len; > vi->dev->stats.tx_packets++; > dev_kfree_skb_any(skb); > } > + return !c; This is for() abuse :) Why is the capacity check in there at all? Surely it's simpler to try to free 2 skbs each time around? for (n = 0; n < 2; n++) { skb = virtqueue_get_buf(vi->svq, &len); if (!skb) break; pr_debug("Sent skb %p\n", skb); vi->dev->stats.tx_bytes += skb->len; vi->dev->stats.tx_packets++; dev_kfree_skb_any(skb); } > static int xmit_skb(struct virtnet_info *vi, struct sk_buff *skb) > @@ -574,8 +582,8 @@ static netdev_tx_t start_xmit(struct sk_buff *skb, struct net_device *dev) > struct virtnet_info *vi = netdev_priv(dev); > int capacity; > > - /* Free up any pending old buffers before queueing new ones. */ > - free_old_xmit_skbs(vi); > + /* Free enough pending old buffers to enable queueing new ones. */ > + free_old_xmit_skbs(vi, 2+MAX_SKB_FRAGS); > > /* Try to transmit */ > capacity = xmit_skb(vi, skb); > @@ -609,9 +617,7 @@ static netdev_tx_t start_xmit(struct sk_buff *skb, struct net_device *dev) > netif_stop_queue(dev); > if (unlikely(!virtqueue_enable_cb_delayed(vi->svq))) { > /* More just got used, free them then recheck. */ > - free_old_xmit_skbs(vi); > - capacity = virtqueue_get_capacity(vi->svq); > - if (capacity >= 2+MAX_SKB_FRAGS) { > + if (!likely(free_old_xmit_skbs(vi, 2+MAX_SKB_FRAGS))) { This extra argument to free_old_xmit_skbs seems odd, unless you have future plans? Thanks, Rusty.