linux-kernel - Re: [RFC PATCH net-next 03/12] vhost_net: introduce vhost_has_more

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Date:   Tue, 22 May 2018 20:31:40 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Jesse Brandeburg <jesse.brandeburg@...el.com>
Cc:     mst@...hat.com, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        virtualization@...ts.linux-foundation.org
Subject: Re: [RFC PATCH net-next 03/12] vhost_net: introduce
 vhost_has_more_pkts()



On 2018年05月22日 00:39, Jesse Brandeburg wrote:
> On Mon, 21 May 2018 17:04:24 +0800 Jason wrote:
>> Signed-off-by: Jason Wang <jasowang@...hat.com>
>> ---
>>   drivers/vhost/net.c | 12 +++++++++---
>>   1 file changed, 9 insertions(+), 3 deletions(-)
>>
>> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
>> index de544ee..4ebac76 100644
>> --- a/drivers/vhost/net.c
>> +++ b/drivers/vhost/net.c
>> @@ -485,6 +485,13 @@ static bool vhost_exceeds_weight(int pkts, int total_len)
>>   	       unlikely(pkts >= VHOST_NET_PKT_WEIGHT);
>>   }
>>   
>> +static bool vhost_has_more_pkts(struct vhost_net *net,
>> +				struct vhost_virtqueue *vq)
>> +{
>> +	return !vhost_vq_avail_empty(&net->dev, vq) &&
>> +	       likely(!vhost_exceeds_maxpend(net));
> This really seems like mis-use of likely/unlikely, in the middle of a
> sequence of operations that will always be run when this function is
> called.  I think you should remove the likely from this helper,
> especially, and control the branch from the branch point.

Yes, so I'm consider to make it a macro in next version.

>
>
>> +}
>> +
>>   /* Expects to be always run from workqueue - which acts as
>>    * read-size critical section for our kind of RCU. */
>>   static void handle_tx(struct vhost_net *net)
>> @@ -578,8 +585,7 @@ static void handle_tx(struct vhost_net *net)
>>   		}
>>   		total_len += len;
>>   		if (total_len < VHOST_NET_WEIGHT &&
>> -		    !vhost_vq_avail_empty(&net->dev, vq) &&
>> -		    likely(!vhost_exceeds_maxpend(net))) {
>> +		    vhost_has_more_pkts(net, vq)) {
> Yes, I know it came from here, but likely/unlikely are for branch
> control, so they should encapsulate everything inside the if, unless
> I'm mistaken.

Ok.

>
>>   			msg.msg_flags |= MSG_MORE;
>>   		} else {
>>   			msg.msg_flags &= ~MSG_MORE;
>> @@ -605,7 +611,7 @@ static void handle_tx(struct vhost_net *net)
>>   		else
>>   			vhost_zerocopy_signal_used(net, vq);
>>   		vhost_net_tx_packet(net);
>> -		if (unlikely(vhost_exceeds_weight(++sent_pkts, total_len))) {
>> +		if (vhost_exceeds_weight(++sent_pkts, total_len)) {
> You should have kept the unlikely here, and not had it inside the
> helper (as per the previous patch.  Also, why wasn't this change part
> of the previous patch?

Yes, will squash the above into previous one.

Thanks

>
>>   			vhost_poll_queue(&vq->poll);
>>   			break;
>>   		}