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

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Date:   Fri, 11 Nov 2016 12:10:44 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     "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 11:31, 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.
> It runs on the same CPU, right? Otherwise we should use skb_array...
>

There could be multiple senders technically. Will try skb_array and see 
if there's any difference.

Thanks