netdev - Re: [PATCH RFC 1/2] virtio-net: bql support

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Date:   Mon, 7 Jan 2019 10:14:37 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     linux-kernel@...r.kernel.org, maxime.coquelin@...hat.com,
        tiwei.bie@...el.com, wexu@...hat.com, jfreimann@...hat.com,
        "David S. Miller" <davem@...emloft.net>,
        virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org
Subject: Re: [PATCH RFC 1/2] virtio-net: bql support


On 2019/1/2 下午9:59, Michael S. Tsirkin wrote:
> On Wed, Jan 02, 2019 at 11:28:43AM +0800, Jason Wang wrote:
>> On 2018/12/31 上午2:45, Michael S. Tsirkin wrote:
>>> On Thu, Dec 27, 2018 at 06:00:36PM +0800, Jason Wang wrote:
>>>> On 2018/12/26 下午11:19, Michael S. Tsirkin wrote:
>>>>> On Thu, Dec 06, 2018 at 04:17:36PM +0800, Jason Wang wrote:
>>>>>> On 2018/12/6 上午6:54, Michael S. Tsirkin wrote:
>>>>>>> When use_napi is set, let's enable BQLs.  Note: some of the issues are
>>>>>>> similar to wifi.  It's worth considering whether something similar to
>>>>>>> commit 36148c2bbfbe ("mac80211: Adjust TSQ pacing shift") might be
>>>>>>> benefitial.
>>>>>> I've played a similar patch several days before. The tricky part is the mode
>>>>>> switching between napi and no napi. We should make sure when the packet is
>>>>>> sent and trakced by BQL,  it should be consumed by BQL as well. I did it by
>>>>>> tracking it through skb->cb.  And deal with the freeze by reset the BQL
>>>>>> status. Patch attached.
>>>>>>
>>>>>> But when testing with vhost-net, I don't very a stable performance,
>>>>> So how about increasing TSQ pacing shift then?
>>>> I can test this. But changing default TCP value is much more than a
>>>> virtio-net specific thing.
>>> Well same logic as wifi applies. Unpredictable latencies related
>>> to radio in one case, to host scheduler in the other.
>>>
>>>>>> it was
>>>>>> probably because we batch the used ring updating so tx interrupt may come
>>>>>> randomly. We probably need to implement time bounded coalescing mechanism
>>>>>> which could be configured from userspace.
>>>>> I don't think it's reasonable to expect userspace to be that smart ...
>>>>> Why do we need time bounded? used ring is always updated when ring
>>>>> becomes empty.
>>>> We don't add used when means BQL may not see the consumed packet in time.
>>>> And the delay varies based on the workload since we count packets not bytes
>>>> or time before doing the batched updating.
>>>>
>>>> Thanks
>>> Sorry I still don't get it.
>>> When nothing is outstanding then we do update the used.
>>> So if BQL stops userspace from sending packets then
>>> we get an interrupt and packets start flowing again.
>> Yes, but how about the cases of multiple flows. That's where I see unstable
>> results.
>>
>>
>>> It might be suboptimal, we might need to tune it but I doubt running
>>> timers is a solution, timer interrupts cause VM exits.
>> Probably not a timer but a time counter (or event byte counter) in vhost to
>> add used and signal guest if it exceeds a value instead of waiting the
>> number of packets.
>>
>>
>> Thanks
> Well we already have VHOST_NET_WEIGHT - is it too big then?


I'm not sure, it might be too big.


>
> And maybe we should expose the "MORE" flag in the descriptor -
> do you think that will help?
>

I don't know. But how a "more" flag can help here?

Thanks


>