linux-kernel - Re: [PATCH 1/3] tuntap: rx batching

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Date:   Fri, 11 Nov 2016 12:28:38 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     John Fastabend <john.fastabend@...il.com>,
        "Michael S. Tsirkin" <mst@...hat.com>
Cc:     netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] tuntap: rx batching



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@...hat.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