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

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Message-ID: <20160518101359.37f5343b@redhat.com>
Date:	Wed, 18 May 2016 10:13:59 +0200
From:	Jesper Dangaard Brouer <brouer@...hat.com>
To:	Jason Wang <jasowang@...hat.com>
Cc:	brouer@...hat.com, Eric Dumazet <eric.dumazet@...il.com>,
	davem@...emloft.net, mst@...hat.com, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH net-next] tuntap: introduce tx skb ring

On Mon, 16 May 2016 15:51:48 +0800
Jason Wang <jasowang@...hat.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