linux-kernel - Re: [PATCH net-next 0/3] vhost: accelerate metadata access through vmap()

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Date:   Wed, 26 Dec 2018 11:59:45 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     kvm@...r.kernel.org, virtualization@...ts.linux-foundation.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH net-next 0/3] vhost: accelerate metadata access through
 vmap()


On 2018/12/25 下午8:52, Michael S. Tsirkin wrote:
> On Tue, Dec 25, 2018 at 06:09:19PM +0800, Jason Wang wrote:
>> On 2018/12/25 上午2:12, Michael S. Tsirkin wrote:
>>> On Mon, Dec 24, 2018 at 04:32:39PM +0800, Jason Wang wrote:
>>>> On 2018/12/14 下午8:33, Michael S. Tsirkin wrote:
>>>>> On Fri, Dec 14, 2018 at 11:42:18AM +0800, Jason Wang wrote:
>>>>>> On 2018/12/13 下午11:27, Michael S. Tsirkin wrote:
>>>>>>> On Thu, Dec 13, 2018 at 06:10:19PM +0800, Jason Wang wrote:
>>>>>>>> Hi:
>>>>>>>>
>>>>>>>> This series tries to access virtqueue metadata through kernel virtual
>>>>>>>> address instead of copy_user() friends since they had too much
>>>>>>>> overheads like checks, spec barriers or even hardware feature
>>>>>>>> toggling.
>>>>>>> Userspace accesses through remapping tricks and next time there's a need
>>>>>>> for a new barrier we are left to figure it out by ourselves.
>>>>>> I don't get here, do you mean spec barriers?
>>>>> I mean the next barrier people decide to put into userspace
>>>>> memory accesses.
>>>>>
>>>>>> It's completely unnecessary for
>>>>>> vhost which is kernel thread.
>>>>> It's defence in depth. Take a look at the commit that added them.
>>>>> And yes quite possibly in most cases we actually have a spec
>>>>> barrier in the validation phase. If we do let's use the
>>>>> unsafe variants so they can be found.
>>>> unsafe variants can only work if you can batch userspace access. This is not
>>>> necessarily the case for light load.
>>> Do we care a lot about the light load? How would you benchmark it?
>>>
>> If we can reduce the latency that's will be more than what we expect.
>>
>> 1 byte TCP_RR shows 1.5%-2% improvement.
> It's nice but not great. E.g. adaptive polling would be
> a better approach to work on latency imho.


Actually this is another advantages of vmap():

For split ring, we will poll avail idx

For packed ring, we will poll wrap counter

Either of which can not be batched.


>
>>>>>> And even if you're right, vhost is not the
>>>>>> only place, there's lots of vmap() based accessing in kernel.
>>>>> For sure. But if one can get by without get user pages, one
>>>>> really should. Witness recently uncovered mess with file
>>>>> backed storage.
>>>> We only pin metadata pages, I don't believe they will be used by any DMA.
>>> It doesn't matter really, if you dirty pages behind the MM back
>>> the problem is there.
>>
>> Ok, but the usual case is anonymous pages, do we use file backed pages for
>> user of vhost?
> Some people use file backed pages for vms.
> Nothing prevents them from using vhost as well.


Ok.


>
>> And even if we use sometime, according to the pointer it's
>> not something that can fix, RFC has been posted to solve this issue.
>>
>> Thanks
> Except it's not broken if we don't to gup + write.
> So yea, wait for rfc to be merged.
>

Yes.

Thanks