Re: [PATCH] vhost-net: add time limitation for tx polling(Internet mail)

[Jason Wang <jasowang@redhat.com>]

On 2018年03月28日 12:01, haibinzhang(张海斌) wrote:
> On 2018年03月27日 19:26, Jason wrote
> On 2018年03月27日 17:12, haibinzhang wrote:
>>> handle_tx() will delay rx for a long time when busy tx polling udp packets
>>> with short length(ie: 1byte udp payload), because setting VHOST_NET_WEIGHT
>>> takes into account only sent-bytes but no time.
>> Interesting.
>>
>> Looking at vhost_can_busy_poll() it tries to poke pending vhost work and
>> exit the busy loop if it found one. So I believe something block the
>> work queuing. E.g did reverting 8241a1e466cd56e6c10472cac9c1ad4e54bc65db
>> fix the issue?
> "busy tx polling" means using netperf send udp packets with 1 bytes payload(total 47bytes frame lenght),
> and handle_tx() will be busy sending packets continuously.
>
>>>    It's not fair for handle_rx(),
>>> so needs to limit max time of tx polling.
>>>
>>> ---
>>>    drivers/vhost/net.c | 3 ++-
>>>    1 file changed, 2 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
>>> index 8139bc70ad7d..dc9218a3a75b 100644
>>> --- a/drivers/vhost/net.c
>>> +++ b/drivers/vhost/net.c
>>> @@ -473,6 +473,7 @@ static void handle_tx(struct vhost_net *net)
>>>    	struct socket *sock;
>>>    	struct vhost_net_ubuf_ref *uninitialized_var(ubufs);
>>>    	bool zcopy, zcopy_used;
>>> +	unsigned long start = jiffies;
>> Checking jiffies is tricky, need to convert it to ms or whatever others.
>>
>>>    
>>>    	mutex_lock(&vq->mutex);
>>>    	sock = vq->private_data;
>>> @@ -580,7 +581,7 @@ static void handle_tx(struct vhost_net *net)
>>>    		else
>>>    			vhost_zerocopy_signal_used(net, vq);
>>>    		vhost_net_tx_packet(net);
>>> -		if (unlikely(total_len >= VHOST_NET_WEIGHT)) {
>>> +		if (unlikely(total_len >= VHOST_NET_WEIGHT) || unlikely(jiffies - start >= 1)) {
>> How value 1 is determined here? And we need a complete test to make sure
>> this won't affect other use cases.
> We just want <1ms ping latency, but actually we are not sure what value is reasonable.
> We have some test results using netperf before this patch as follow,
>
>      Udp payload    1byte    100bytes    1000bytes    1400bytes
>    Ping avg latency    25ms     10ms       2ms         1.5ms
>
> What is other testcases?

Something like https://patchwork.kernel.org/patch/10151645/.

Btw, you need use time_before() to properly handle jiffies overflow and 
I would also suggest you to try something like #packets limit (e.g 64).

For long term, we definitely need more worker threads.

Thanks

>
>> Another thought is introduce another limit of #packets, but this need
>> benchmark too.
>>
>> Thanks
>>
>>>    			vhost_poll_queue(&vq->poll);
>>>    			break;
>>>    		}
>>