Re: [PATCH net-next] tuntap: introduce tx skb ring

Jesper Dangaard Brouer
2016-05-18T08:14:29Z

On Mon, 16 May 2016 15:51:48 +0800
Jason Wang <jasowang@redhat.com> wrote:

> On 2016年05月16日 11:56, Eric Dumazet wrote:
> > On Mon, 2016-05-16 at 09:17 +0800, Jason Wang wrote:
> >> We used to queue tx packets in sk_receive_queue, this is less
> >> efficient since it requires spinlocks to synchronize between producer
> >> and consumer.
> > ...
> >
> >> struct tun_struct *detached;
> >> + /* reader lock */
> >> + spinlock_t rlock;
> >> + unsigned long tail;
> >> + struct tun_desc tx_descs[TUN_RING_SIZE];
> >> + /* writer lock */
> >> + spinlock_t wlock;
> >> + unsigned long head;
> >> };
> >>
> > Ok, we had these kind of ideas floating around for many other cases,
> > like qdisc, UDP or af_packet sockets...
> >
> > I believe we should have a common set of helpers, not hidden in
> > drivers/net/tun.c but in net/core/skb_ring.c or something, with more
> > flexibility (like the number of slots)
> >
>
> Yes, this sounds good.

I agree. It is sad to see everybody is implementing the same thing,
open coding an array/circular based ring buffer. This kind of code is
hard to maintain and get right with barriers etc. We can achieve the
same performance with a generic implementation, by inlining the help
function calls.

I implemented an array based Lock-Free/cmpxchg based queue, that you
could be inspired by, see:
https://github.com/netoptimizer/prototype-kernel/blob/master/kernel/include/linux/alf_queue.h

The main idea behind my implementation is bulking, to amortize the
locked cmpxchg operation. You might not need it now, but I expect we
need it in the future.

You cannot use my alf_queue directly as your "struct tun_desc" is
larger than one-pointer (which the alf_queue works with). But it
should be possible to extend to handle larger "objects".

Maybe Steven Rostedt have an even better ring queue implementation
already avail in the kernel?

--
Best regards,
Jesper Dangaard Brouer
MSc.CS, Principal Kernel Engineer at Red Hat
Author of http://www.iptv-analyzer.org
LinkedIn: http://www.linkedin.com/in/brouer