Re: [PATCH v6 0/3] skb_array: array based FIFO for skbs

[Jason Wang]

On 2016å06æ02æ 12:51, David Miller wrote:
> From: "Michael S. Tsirkin" <mst@xxxxxxxxxx>
> Date: Wed, 1 Jun 2016 15:54:34 +0300
>
> > This is in response to the proposal by Jason to make tun
> > rx packet queue lockless using a circular buffer.
> > My testing seems to show that at least for the common usecase
> > in networking, which isn't lockless, circular buffer
> > with indices does not perform that well, because
> > each index access causes a cache line to bounce between
> > CPUs, and index access causes stalls due to the dependency.
> >
> > By comparison, an array of pointers where NULL means invalid
> > and !NULL means valid, can be updated without messing up barriers
> > at all and does not have this issue.
> >
> > On the flip side, cache pressure may be caused by using large queues.
> > tun has a queue of 1000 entries by default and that's 8K.
> > At this point I'm not sure this can be solved efficiently.
> > The correct solution might be sizing the queues appropriately.
> >
> > Here's an implementation of this idea: it can be used more
> > or less whenever sk_buff_head can be used, except you need
> > to know the queue size in advance.
>   ...
>
> I have no fundamental issues with this piece of infrastructure, but when
> it gets included I want this series to include at least one use case.
>
> This can be an adaptation of Jason's tun rx packet queue changes, or
> similar.
>
> Thanks.

Right, I'm working on using skb array for tun, will post the patch in 
the following days.

Thanks