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SubjectRe: [PATCH 1/3] tuntap: rx batching
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On 2016年11月12日 00:20, Michael S. Tsirkin wrote:
> On Fri, Nov 11, 2016 at 12:28:38PM +0800, Jason Wang wrote:
>>
>> On 2016年11月11日 12:17, John Fastabend wrote:
>>> On 16-11-10 07:31 PM, Michael S. Tsirkin wrote:
>>>>> On Fri, Nov 11, 2016 at 10:07:44AM +0800, Jason Wang wrote:
>>>>>>>
>>>>>>> On 2016年11月10日 00:38, Michael S. Tsirkin wrote:
>>>>>>>>> On Wed, Nov 09, 2016 at 03:38:31PM +0800, Jason Wang wrote:
>>>>>>>>>>> Backlog were used for tuntap rx, but it can only process 1 packet at
>>>>>>>>>>> one time since it was scheduled during sendmsg() synchronously in
>>>>>>>>>>> process context. This lead bad cache utilization so this patch tries
>>>>>>>>>>> to do some batching before call rx NAPI. This is done through:
>>>>>>>>>>>
>>>>>>>>>>> - accept MSG_MORE as a hint from sendmsg() caller, if it was set,
>>>>>>>>>>> batch the packet temporarily in a linked list and submit them all
>>>>>>>>>>> once MSG_MORE were cleared.
>>>>>>>>>>> - implement a tuntap specific NAPI handler for processing this kind of
>>>>>>>>>>> possible batching. (This could be done by extending backlog to
>>>>>>>>>>> support skb like, but using a tun specific one looks cleaner and
>>>>>>>>>>> easier for future extension).
>>>>>>>>>>>
>>>>>>>>>>> Signed-off-by: Jason Wang<jasowang@redhat.com>
>>>>>>>>> So why do we need an extra queue?
>>>>>>> The idea was borrowed from backlog to allow some kind of bulking and avoid
>>>>>>> spinlock on each dequeuing.
>>>>>>>
>>>>>>>>> This is not what hardware devices do.
>>>>>>>>> How about adding the packet to queue unconditionally, deferring
>>>>>>>>> signalling until we get sendmsg without MSG_MORE?
>>>>>>> Then you need touch spinlock when dequeuing each packet.
>>> Random thought, I have a cmpxchg ring I am using for the qdisc work that
>>> could possibly replace the spinlock implementation. I haven't figured
>>> out the resizing API yet because I did not need it but I assume it could
>>> help here and let you dequeue multiple skbs in one operation.
>>>
>>> I can post the latest version if useful or an older version is
>>> somewhere on patchworks as well.
>>>
>>> .John
>>>
>>>
>> Look useful here, and I can compare the performance if you post.
>>
>> A question is can we extend the skb_array to support that?
>>
>> Thanks
> I'd like to start with simple patch adding napi with one queue, then add
> optimization patches on top.

The point is tun is using backlog who uses two queues (process_queue and
input_pkt_queue).

How about something like:

1) NAPI support with skb_array
2) MSG_MORE support
3) other optimizations on top

?

>
> One issue that comes to mind is that write queue limits
> are byte based, they do not count packets unlike tun rx queue.

I'm not sure I get the issue, write queue is not exported and only used
for batching. We probably need an internal limit in tun to avoid OOM
attacker from guest.

Thanks

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