netdev - Re: [PATCH V3 2/2] vhost_net: conditionally enable tx polling

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Date:	Wed, 8 Jun 2016 14:48:18 +0800
From:	Jason Wang <jasowang@...hat.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	kvm@...r.kernel.org, virtualization@...ts.linux-foundation.org,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH V3 2/2] vhost_net: conditionally enable tx polling



On 2016年06月07日 20:26, Michael S. Tsirkin wrote:
> On Wed, Jun 01, 2016 at 01:56:34AM -0400, Jason Wang wrote:
>> We always poll tx for socket, this is sub optimal since:
>>
>> - it will be only used when we exceed the sndbuf of the socket.
>> - since we use two independent polls for tx and vq, this will slightly
>>    increase the waitqueue traversing time and more important, vhost
>>    could not benefit from commit
>>    9e641bdcfa4ef4d6e2fbaa59c1be0ad5d1551fd5 ("net-tun: restructure
>>    tun_do_read for better sleep/wakeup efficiency") even if we've
>>    stopped rx polling during handle_rx since tx poll were still left in
>>    the waitqueue.
>>
>> Fix this by conditionally enable tx polling only when -EAGAIN were
>> met.
>>
>> Test shows about 8% improvement on guest rx pps.
>>
>> Before: ~1350000
>> After:  ~1460000
>>
>> Signed-off-by: Jason Wang <jasowang@...hat.com>
>> ---
>>   drivers/vhost/net.c | 3 +++
>>   1 file changed, 3 insertions(+)
>>
>> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
>> index 1d3e45f..e75ffcc 100644
>> --- a/drivers/vhost/net.c
>> +++ b/drivers/vhost/net.c
>> @@ -378,6 +378,7 @@ static void handle_tx(struct vhost_net *net)
>>   		goto out;
>>   
>>   	vhost_disable_notify(&net->dev, vq);
>> +	vhost_net_disable_vq(net, vq);
>>   
>>   	hdr_size = nvq->vhost_hlen;
>>   	zcopy = nvq->ubufs;
>> @@ -459,6 +460,8 @@ static void handle_tx(struct vhost_net *net)
>>   					% UIO_MAXIOV;
>>   			}
>>   			vhost_discard_vq_desc(vq, 1);
>> +			if (err == -EAGAIN)
>> +				vhost_net_enable_vq(net, vq);
>>   			break;
>>   		}
>>   		if (err != len)
> This seems rather risky. What if TX failed for some other reason?
> Polling won't ever be re-enabled ...
>

But why we need to enable tx poll in this case? Even if we enable it, we 
wont' get any wakeup.

>> -- 
>> 1.8.3.1