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SubjectRe: [PATCH 6/6] vhost_net: don't poll on -EFAULT
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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