Re: [PATCH net] vhost_net: fix high cpu load when sendmsg fails

[Jason Wang <jasowang@redhat.com>]

On 2020/12/11 下午3:37, wangyunjian wrote:
>> -----Original Message-----
>> From: Jason Wang [mailto:jasowang@redhat.com]
>> Sent: Friday, December 11, 2020 10:53 AM
>> To: wangyunjian <wangyunjian@huawei.com>; Michael S. Tsirkin
>> <mst@redhat.com>
>> Cc: virtualization@lists.linux-foundation.org; netdev@vger.kernel.org; Lilijun
>> (Jerry) <jerry.lilijun@huawei.com>; chenchanghu <chenchanghu@huawei.com>;
>> xudingke <xudingke@huawei.com>
>> Subject: Re: [PATCH net] vhost_net: fix high cpu load when sendmsg fails
>>
>>
>> On 2020/12/9 下午9:27, wangyunjian wrote:
>>>> -----Original Message-----
>>>> From: Michael S. Tsirkin [mailto:mst@redhat.com]
>>>> Sent: Wednesday, December 9, 2020 8:50 PM
>>>> To: wangyunjian <wangyunjian@huawei.com>
>>>> Cc: jasowang@redhat.com; virtualization@lists.linux-foundation.org;
>>>> netdev@vger.kernel.org; Lilijun (Jerry) <jerry.lilijun@huawei.com>;
>>>> chenchanghu <chenchanghu@huawei.com>; xudingke
>> <xudingke@huawei.com>
>>>> Subject: Re: [PATCH net] vhost_net: fix high cpu load when sendmsg
>>>> fails
>>>>
>>>> On Wed, Dec 09, 2020 at 07:48:24PM +0800, wangyunjian wrote:
>>>>> From: Yunjian Wang <wangyunjian@huawei.com>
>>>>>
>>>>> Currently we break the loop and wake up the vhost_worker when
>>>>> sendmsg fails. When the worker wakes up again, we'll meet the same
>>>>> error. This will cause high CPU load. To fix this issue, we can skip
>>>>> this description by ignoring the error.
>>>>>
>>>>> Signed-off-by: Yunjian Wang <wangyunjian@huawei.com>
>>>>> ---
>>>>>    drivers/vhost/net.c | 24 +++++-------------------
>>>>>    1 file changed, 5 insertions(+), 19 deletions(-)
>>>>>
>>>>> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c index
>>>>> 531a00d703cd..ac950b1120f5 100644
>>>>> --- a/drivers/vhost/net.c
>>>>> +++ b/drivers/vhost/net.c
>>>>> @@ -829,14 +829,8 @@ static void handle_tx_copy(struct vhost_net
>>>>> *net, struct socket *sock)
>>>>>
>>>>>    		/* TODO: Check specific error and bomb out unless ENOBUFS?
>> */
>>>>>    		err = sock->ops->sendmsg(sock, &msg, len);
>>>>> -		if (unlikely(err < 0)) {
>>>>> -			vhost_discard_vq_desc(vq, 1);
>>>>> -			vhost_net_enable_vq(net, vq);
>>>>> -			break;
>>>>> -		}
>>>>> -		if (err != len)
>>>>> -			pr_debug("Truncated TX packet: len %d != %zd\n",
>>>>> -				 err, len);
>>>>> +		if (unlikely(err < 0 || err != len))
>>>>> +			vq_err(vq, "Fail to sending packets err : %d, len : %zd\n",
>> err,
>>>>> +len);
>>>>>    done:
>>>>>    		vq->heads[nvq->done_idx].id = cpu_to_vhost32(vq, head);
>>>>>    		vq->heads[nvq->done_idx].len = 0;
>>>> One of the reasons for sendmsg to fail is ENOBUFS.
>>>> In that case for sure we don't want to drop packet.
>>> Now the function tap_sendmsg()/tun_sendmsg() don't return ENOBUFS.
>>
>> I think not, it can happen if we exceeds sndbuf. E.g see tun_alloc_skb().
> This patch 'net: add alloc_skb_with_frags() helper' modifys the return value
> of sock_alloc_send_pskb() from -ENOBUFS to -EAGAIN when we exceeds sndbuf.
> So the return value of tun_alloc_skb has been changed.


Ok.


>
> We don't drop packet if the reasons for sendmsg to fail is EAGAIN.
> How about this?


It should work.

Btw, the patch doesn't add the head to the used ring. This may confuses 
the driver.

Thanks


>
> Thanks
>
>> Thanks
>>
>>
>>>> There could be other transient errors.
>>>> Which error did you encounter, specifically?
>>> Currently a guest vm send a skb which length is more than 64k.
>>> If virtio hdr is wrong, the problem will also be triggered.
>>>
>>> Thanks
>>>
>>>>> @@ -925,19 +919,11 @@ static void handle_tx_zerocopy(struct
>>>>> vhost_net *net, struct socket *sock)
>>>>>
>>>>>    		/* TODO: Check specific error and bomb out unless ENOBUFS?
>> */
>>>>>    		err = sock->ops->sendmsg(sock, &msg, len);
>>>>> -		if (unlikely(err < 0)) {
>>>>> -			if (zcopy_used) {
>>>>> +		if (unlikely(err < 0 || err != len)) {
>>>>> +			if (zcopy_used && err < 0)
>>>>>    				vhost_net_ubuf_put(ubufs);
>>>>> -				nvq->upend_idx = ((unsigned)nvq->upend_idx - 1)
>>>>> -					% UIO_MAXIOV;
>>>>> -			}
>>>>> -			vhost_discard_vq_desc(vq, 1);
>>>>> -			vhost_net_enable_vq(net, vq);
>>>>> -			break;
>>>>> +			vq_err(vq, "Fail to sending packets err : %d, len : %zd\n",
>> err,
>>>>> +len);
>>>>>    		}
>>>>> -		if (err != len)
>>>>> -			pr_debug("Truncated TX packet: "
>>>>> -				 " len %d != %zd\n", err, len);
>>>>>    		if (!zcopy_used)
>>>>>    			vhost_add_used_and_signal(&net->dev, vq, head, 0);
>>>>>    		else
>>>>> --
>>>>> 2.23.0