From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jason Wang Subject: Re: [PATCH] vhost: poll vhost_net only when tx notification is enabled Date: Fri, 28 Feb 2014 15:49:22 +0800 Message-ID: <53103F82.7000904@redhat.com> References: <530DB1C9.2060106@huawei.com> <20140226111645.GC5236@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Cc: davem@davemloft.net, netdev@vger.kernel.org, KVM list , zhangjie14@huawei.com To: "Michael S. Tsirkin" , Qin Chuanyu Return-path: Received: from mx1.redhat.com ([209.132.183.28]:59702 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750844AbaB1HuQ (ORCPT ); Fri, 28 Feb 2014 02:50:16 -0500 In-Reply-To: <20140226111645.GC5236@redhat.com> Sender: kvm-owner@vger.kernel.org List-ID: On 02/26/2014 07:16 PM, Michael S. Tsirkin wrote: > Please see MAINTAINERS and copy all relevant lists. > > On Wed, Feb 26, 2014 at 05:20:09PM +0800, Qin Chuanyu wrote: >> guest kick host base on avail_ring flags value and get perfermance > typo > >> improved, vhost_zerocopy_callback could do the same thing. As >> virtqueue_enable_cb need one more check after modifying the value of >> avail_ring flags, vhost also need do the same thing after >> vhost_enable_notify. >> >> test result list as below: >> guest and host: suse11sp3, netperf, intel CPU 2.4GHz >> +------+----------+--------+----------+--------+--------+---------+ >> | | old | new | >> +------+----------+--------+----------+--------+--------+---------+ >> | UDP | Gbit/s | PPS |CPU idle% | Gbit/s | PPS |CPU idle%| >> | 256 | 0.74805 | 321309 | 87.16 | 0.77933| 334743 | 90.71 | >> | 512 | 1.42 | 328475 | 87.03 | 1.44 | 333550 | 90.43 | >> | 1024 | 2.79 | 334426 | 89.09 | 2.81 | 336986 | 89.55 | >> | 1460 | 3.71 | 316215 | 87.53 | 4.02 | 342325 | 89.58 | >> +------+----------+--------+----------+--------+--------+---------+ >> >> Signed-off-by: Chuanyu Qin > It's an interesting optimization, thanks! > However, it looks like this might delay > updating used ring indefinitely if we are > unlucky. Some guests (e.g. windows) > tend to crash if this happens. Looks like it does not change this. When tx notification is disabled, it means handle_tx() is running which will update the used ring before each tx descriptor fetching. And the patch does used updating after each time when tx notification is enabled. > > Maybe use a new flag for this? > > It also looks like there are potential race conditions below. > >> --- >> drivers/vhost/net.c | 13 ++++++++++++- >> 1 files changed, 12 insertions(+), 1 deletions(-) >> >> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c >> index a0fa5de..a90f51b 100644 >> --- a/drivers/vhost/net.c >> +++ b/drivers/vhost/net.c >> @@ -315,6 +315,10 @@ static void vhost_zerocopy_callback(struct >> ubuf_info *ubuf, bool success) >> VHOST_DMA_DONE_LEN : VHOST_DMA_FAILED_LEN; >> cnt = vhost_net_ubuf_put(ubufs); >> >> + /* make sure len has been updated because handle_tx would use it >> + * and used_flags should also been checked. >> + */ >> + smp_mb(); >> /* >> * Trigger polling thread if guest stopped submitting new buffers: >> * in this case, the refcount after decrement will eventually reach 1. > this barrier is very suspect. > >> @@ -322,7 +326,8 @@ static void vhost_zerocopy_callback(struct >> ubuf_info *ubuf, bool success) >> * (the value 16 here is more or less arbitrary, it's tuned to trigger >> * less than 10% of times). >> */ >> - if (cnt <= 1 || !(cnt % 16)) >> + if ((!(vq->used_flags & VRING_USED_F_NO_NOTIFY)) >> + && (cnt <= 1 || !(cnt % 16))) >> vhost_poll_queue(&vq->poll); >> >> rcu_read_unlock_bh(); > looks like a potential race to me Could you please explain the race a little bit more? >> @@ -386,6 +391,12 @@ static void handle_tx(struct vhost_net *net) >> vhost_disable_notify(&net->dev, vq); >> continue; >> } >> + /* there might skb been freed between last >> + * vhost_zerocopy_signal_used and vhost_enable_notify, >> + * so one more check is needed. >> + */ >> + if (zcopy) >> + vhost_zerocopy_signal_used(net, vq); > >> break; >> } >> if (in) { >> -- >> 1.7.3.1.msysgit.0 >> > -- > To unsubscribe from this list: send the line "unsubscribe kvm" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html