Messages in this thread Patch in this message | | | Date | Tue, 17 Apr 2012 18:46:25 +0800 | From | Jason Wang <> | Subject | Re: [PATCH 6/6] vhost_net: don't poll on -EFAULT |
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On 04/17/2012 06:18 PM, Michael S. Tsirkin wrote: > On Tue, Apr 17, 2012 at 02:30:27PM +0800, Jason Wang wrote: >> > On 04/17/2012 02:07 PM, Michael S. Tsirkin wrote: >>> > >On Tue, Apr 17, 2012 at 01:54:55PM +0800, Jason Wang wrote: >>>> > >>On 04/17/2012 12:57 PM, Michael S. Tsirkin wrote: >>>>> > >>>On Tue, Apr 17, 2012 at 11:27:01AM +0800, Jason Wang wrote: >>>>>> > >>>>On 04/16/2012 09:39 PM, Michael S. Tsirkin wrote: >>>>>>> > >>>>>On Mon, Apr 16, 2012 at 04:28:10PM +0800, Jason Wang wrote: >>>>>>>>> > >>>>>>> On 04/16/2012 03:16 PM, Michael S. Tsirkin wrote: >>>>>>>>>> > >>>>>>>> >On Mon, Apr 16, 2012 at 02:08:33PM +0800, Jason Wang wrote: >>>>>>>>>>> > >>>>>>>>> >>Currently, we restart tx polling unconditionally when sendmsg() >>>>>>>>>>> > >>>>>>>>> >>fails. This would cause unnecessary wakeups of vhost wokers as it's >>>>>>>>>>> > >>>>>>>>> >>only needed when the socket send buffer were exceeded. >>>>>>>>>> > >>>>>>>> >Why is this a problem? >>>>>>>>> > >>>>>>> > This issue is when guest driver is able to hit the >>>>>>>> > >>>>>>-EFAULT, vhost >>>>>>>>> > >>>>>>> discard the the descriptor and restart the polling. This would wake >>>>>>>>> > >>>>>>> vhost thread and repeat the loop again which waste cpu. >>>>>>> > >>>>>Does same thing happen if we get an error from copy from user? >>>>>>> > >>>>> >>>>>> > >>>>Right, so do you think it makes sense that we only restart polling >>>>>> > >>>>on -EAGAIN or -ENOBUFS? >>>>> > >>>Sounds OK. BTW how do you test this? >>>>> > >>> >>>> > >>Not very hard, w/o this patch, we can see almost 100% cpu >>>> > >>utilization for vhost thread if guest hit EFAULT or EINVAL. With >>>> > >>this patch, the cpu utilization should be very low I think. >>> > >Yes but do you have a test that makes guest hit EFAULT or EINVAL? >> > >> > Looks like we can do this by supplying an invalid hdr_len in vnet >> > header as tap does the check for this. > Ah so you patched qemu to do this? Cool. Can you post the patch for testing pls? > No, I mean patch the guest driver like this:
diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c index 019da01..6e2f487 100644 --- a/drivers/net/virtio_net.c +++ b/drivers/net/virtio_net.c @@ -582,7 +582,7 @@ static int xmit_skb(struct virtnet_info *vi, struct sk_buff *skb) }
if (skb_is_gso(skb)) { - hdr->hdr.hdr_len = skb_headlen(skb); + hdr->hdr.hdr_len = 65535; hdr->hdr.gso_size = skb_shinfo(skb)->gso_size; if (skb_shinfo(skb)->gso_type & SKB_GSO_TCPV4) hdr->hdr.gso_type = VIRTIO_NET_HDR_GSO_TCPV4; btw, If we still choose to drop the packet, we can hit -EFAULT by send a descriptor with a large number of pages.
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