Re: [PATCH net-next] vhost_net: conditionally enable tx polling

[Jason Wang <jasowang@redhat.com>]

On 2017年11月01日 23:03, Michael S. Tsirkin wrote:
> On Wed, Nov 01, 2017 at 08:51:36PM +0800, Jason Wang wrote:
>>
>> On 2017年11月01日 00:36, Michael S. Tsirkin wrote:
>>> On Tue, Oct 31, 2017 at 06:27:20PM +0800, Jason Wang wrote:
>>>> We always poll tx for socket, this is sub optimal since:
>>>>
>>>> - we only want to be notified when sndbuf is available
>>>> - this will slightly increase the waitqueue traversing time and more
>>>>     important, vhost could not benefit from commit
>>>>     commit 9e641bdcfa4e
>>>>     ("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.
>>>>
>>>> Pktgen from a remote host to VM over mlx4 shows 5.5% improvements on
>>>> rx PPS. (from 1.27Mpps to 1.34Mpps)
>>>>
>>>> Cc: Wei Xu <wexu@redhat.com>
>>>> Cc: Matthew Rosato <mjrosato@linux.vnet.ibm.com>
>>>> Signed-off-by: Jason Wang <jasowang@redhat.com>
>>>> ---
>>> Now that vhost_poll_stop happens on data path
>>> a lot, I'd say
>>>           if (poll->wqh)
>>> there should be unlikely().
>> It has been there since 8241a1e466cd ("vhost_net: stop polling socket during
>> rx processing"). So it will be used for rx path too which unlikely() does
>> not work as well as the case in tx.
> Worth testing, does not have to block this patch.

Ok.

>>>
>>>>    drivers/vhost/net.c | 11 ++++++++---
>>>>    1 file changed, 8 insertions(+), 3 deletions(-)
>>>>
>>>> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
>>>> index 68677d9..286c3e4 100644
>>>> --- a/drivers/vhost/net.c
>>>> +++ b/drivers/vhost/net.c
>>>> @@ -471,6 +471,7 @@ 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;
>>>> @@ -556,6 +557,8 @@ 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)
>>> I would probably just enable it unconditionally here. Why not?
>>>
>> I thought we only care about the case of tun_sock_write_space() and for the
>> errors other than -EAGAIN, they have nothing to do with polling.
> We could thinkably get ENOMEM I guess. If we miss a code things
> get stuck - It's just easier not to add extra code IMHO.

ENOBUFS actually. I agree to remove the specific checking of -EAGAIN here.

>
>>>> @@ -1145,9 +1148,11 @@ static long vhost_net_set_backend(struct vhost_net *n, unsigned index, int fd)
>>>>    		r = vhost_vq_init_access(vq);
>>>>    		if (r)
>>>>    			goto err_used;
>>>> -		r = vhost_net_enable_vq(n, vq);
>>>> -		if (r)
>>>> -			goto err_used;
>>>> +		if (index == VHOST_NET_VQ_RX) {
>>>> +			r = vhost_net_enable_vq(n, vq);
>>>> +			if (r)
>>>> +				goto err_used;
>>>> +		}
>>>>    		oldubufs = nvq->ubufs;
>>>>    		nvq->ubufs = ubufs;
>>> This last chunk seems questionable. If queue has stuff in it
>>> when we connect the backend, we'll miss a wakeup.
>>> I suspect this can happen during migration.
>> Unless qemu pass a tap which s already had pending tx packets.
>>
>> I can remove this chuck, but if guest does not transmit any packet, rx can't
>> benefit from this.
>>
>> Thanks
> Not sure I understand the last sentence. vhost will stay
> polling the socket - why is that a problem?

No function problem at all. If guest does not transmit any packet, the 
patch does not have any effect.

Will remove this chunk in next version.

Thanks

>
>>>
>>>> -- 
>>>> 2.7.4
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