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

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Date:	Mon, 16 Apr 2012 16:28:10 +0800
From:	Jason Wang <jasowang@...hat.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
CC:	netdev@...r.kernel.org, xma@...ibm.com, davem@...emloft.net,
	linux-kernel@...r.kernel.org, ebiederm@...ssion.com
Subject: Re: [PATCH 6/6] vhost_net: don't poll on -EFAULT

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.

Another possible solution is don't discard the descriptor.
>
>> Fix this by
>> restart the tx polling only when sendmsg() returns value other than
>> -EFAULT.
>>
>> Signed-off-by: Jason Wang<jasowang@...hat.com>
>> ---
>>   drivers/vhost/net.c |    3 ++-
>>   1 files changed, 2 insertions(+), 1 deletions(-)
>>
>> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
>> index 29abd65..035fa95 100644
>> --- a/drivers/vhost/net.c
>> +++ b/drivers/vhost/net.c
>> @@ -262,7 +262,8 @@ static void handle_tx(struct vhost_net *net)
>>   					UIO_MAXIOV;
>>   			}
>>   			vhost_discard_vq_desc(vq, 1);
>> -			tx_poll_start(net, sock);
>> +			if (err != -EFAULT)
>> +				tx_poll_start(net, sock);
>>   			break;
>>   		}
>>   		if (err != len)
> --
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