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

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Message-ID: <f1a80199-ccf2-850b-1249-31f8f920ede3@redhat.com>
Date:   Thu, 29 Mar 2018 10:00:51 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     haibinzhang(张海斌) 
        <haibinzhang@...cent.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "virtualization@...ts.linux-foundation.org" 
        <virtualization@...ts.linux-foundation.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        lidongchen(陈立东) <lidongchen@...cent.com>,
        yunfangtai(台运方) <yunfangtai@...cent.com>
Subject: Re: [PATCH] vhost-net: add time limitation for tx polling(Internet
 mail)



On 2018年03月28日 23:31, Michael S. Tsirkin wrote:
> On Wed, Mar 28, 2018 at 02:37:04PM +0800, Jason Wang wrote:
>>
>> 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).
> Maybe a ring size?

Yes or a factor of ring size.

>
>> For long term, we definitely need more worker threads.
>>
>> Thanks
> Only helps when you have spare CPUs.

Right.

Thanks

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