From mboxrd@z Thu Jan 1 00:00:00 1970 From: Yang Zhang Subject: Re: [PATCH V2 3/3] vhost_net: basic polling support Date: Thu, 21 Jan 2016 14:39:51 +0800 Message-ID: <56A07D37.4000408@gmail.com> References: <1448951985-12385-1-git-send-email-jasowang@redhat.com> <1448951985-12385-4-git-send-email-jasowang@redhat.com> <20160120143524.GA27168@redhat.com> <56A03E57.2020400@gmail.com> <20160121064759-mutt-send-email-mst@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii"; Format="flowed" Content-Transfer-Encoding: 7bit Cc: netdev@vger.kernel.org, linux-kernel@vger.kernel.org, kvm@vger.kernel.org, virtualization@lists.linux-foundation.org To: "Michael S. Tsirkin" Return-path: In-Reply-To: <20160121064759-mutt-send-email-mst@redhat.com> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: virtualization-bounces@lists.linux-foundation.org Errors-To: virtualization-bounces@lists.linux-foundation.org List-Id: netdev.vger.kernel.org On 2016/1/21 13:13, Michael S. Tsirkin wrote: > On Thu, Jan 21, 2016 at 10:11:35AM +0800, Yang Zhang wrote: >> On 2016/1/20 22:35, Michael S. Tsirkin wrote: >>> On Tue, Dec 01, 2015 at 02:39:45PM +0800, Jason Wang wrote: >>>> This patch tries to poll for new added tx buffer or socket receive >>>> queue for a while at the end of tx/rx processing. The maximum time >>>> spent on polling were specified through a new kind of vring ioctl. >>>> >>>> Signed-off-by: Jason Wang >>>> --- >>>> drivers/vhost/net.c | 72 ++++++++++++++++++++++++++++++++++++++++++---- >>>> drivers/vhost/vhost.c | 15 ++++++++++ >>>> drivers/vhost/vhost.h | 1 + >>>> include/uapi/linux/vhost.h | 11 +++++++ >>>> 4 files changed, 94 insertions(+), 5 deletions(-) >>>> >>>> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c >>>> index 9eda69e..ce6da77 100644 >>>> --- a/drivers/vhost/net.c >>>> +++ b/drivers/vhost/net.c >>>> @@ -287,6 +287,41 @@ static void vhost_zerocopy_callback(struct ubuf_info *ubuf, bool success) >>>> rcu_read_unlock_bh(); >>>> } >>>> >>>> +static inline unsigned long busy_clock(void) >>>> +{ >>>> + return local_clock() >> 10; >>>> +} >>>> + >>>> +static bool vhost_can_busy_poll(struct vhost_dev *dev, >>>> + unsigned long endtime) >>>> +{ >>>> + return likely(!need_resched()) && >>>> + likely(!time_after(busy_clock(), endtime)) && >>>> + likely(!signal_pending(current)) && >>>> + !vhost_has_work(dev) && >>>> + single_task_running(); >>>> +} >>>> + >>>> +static int vhost_net_tx_get_vq_desc(struct vhost_net *net, >>>> + struct vhost_virtqueue *vq, >>>> + struct iovec iov[], unsigned int iov_size, >>>> + unsigned int *out_num, unsigned int *in_num) >>>> +{ >>>> + unsigned long uninitialized_var(endtime); >>>> + >>>> + if (vq->busyloop_timeout) { >>>> + preempt_disable(); >>>> + endtime = busy_clock() + vq->busyloop_timeout; >>>> + while (vhost_can_busy_poll(vq->dev, endtime) && >>>> + !vhost_vq_more_avail(vq->dev, vq)) >>>> + cpu_relax(); >>>> + preempt_enable(); >>>> + } >>> >>> Isn't there a way to call all this after vhost_get_vq_desc? >>> First, this will reduce the good path overhead as you >>> won't have to play with timers and preemption. >>> >>> Second, this will reduce the chance of a pagefault on avail ring read. >>> >>>> + >>>> + return vhost_get_vq_desc(vq, vq->iov, ARRAY_SIZE(vq->iov), >>>> + out_num, in_num, NULL, NULL); >>>> +} >>>> + >>>> /* Expects to be always run from workqueue - which acts as >>>> * read-size critical section for our kind of RCU. */ >>>> static void handle_tx(struct vhost_net *net) >>>> @@ -331,10 +366,9 @@ static void handle_tx(struct vhost_net *net) >>>> % UIO_MAXIOV == nvq->done_idx)) >>>> break; >>>> >>>> - head = vhost_get_vq_desc(vq, vq->iov, >>>> - ARRAY_SIZE(vq->iov), >>>> - &out, &in, >>>> - NULL, NULL); >>>> + head = vhost_net_tx_get_vq_desc(net, vq, vq->iov, >>>> + ARRAY_SIZE(vq->iov), >>>> + &out, &in); >>>> /* On error, stop handling until the next kick. */ >>>> if (unlikely(head < 0)) >>>> break; >>>> @@ -435,6 +469,34 @@ static int peek_head_len(struct sock *sk) >>>> return len; >>>> } >>>> >>>> +static int vhost_net_peek_head_len(struct vhost_net *net, struct sock *sk) >>> >>> Need a hint that it's rx related in the name. >>> >>>> +{ >>>> + struct vhost_net_virtqueue *nvq = &net->vqs[VHOST_NET_VQ_TX]; >>>> + struct vhost_virtqueue *vq = &nvq->vq; >>>> + unsigned long uninitialized_var(endtime); >>>> + >>>> + if (vq->busyloop_timeout) { >>>> + mutex_lock(&vq->mutex); >>> >>> This appears to be called under vq mutex in handle_rx. >>> So how does this work then? >>> >>> >>>> + vhost_disable_notify(&net->dev, vq); >>> >>> This appears to be called after disable notify >>> in handle_rx - so why disable here again? >>> >>>> + >>>> + preempt_disable(); >>>> + endtime = busy_clock() + vq->busyloop_timeout; >>>> + >>>> + while (vhost_can_busy_poll(&net->dev, endtime) && >>>> + skb_queue_empty(&sk->sk_receive_queue) && >>>> + !vhost_vq_more_avail(&net->dev, vq)) >>>> + cpu_relax(); >>> >>> This seems to mix in several items. >>> RX queue is normally not empty. I don't think >>> we need to poll for that. >> >> I have seen the RX queue is easy to be empty under some extreme conditions >> like lots of small packet. So maybe the check is useful here. > > It's not useful *here*. > If you have an rx packet but no space in the ring, > this will exit immediately. Indeed! > > It might be useful elsewhere but I doubt it - > if rx ring is out of buffers, you are better off > backing out and giving guest some breathing space. > >> -- >> best regards >> yang -- best regards yang