netdev - Re: [RFC PATCH net] net/core: don't increment rx

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Date:	Tue, 26 Jan 2016 16:14:53 -0500
From:	Jarod Wilson <jarod@...hat.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	linux-kernel@...r.kernel.org,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jiri Pirko <jiri@...lanox.com>,
	Daniel Borkmann <daniel@...earbox.net>,
	Tom Herbert <tom@...bertland.com>,
	Jay Vosburgh <j.vosburgh@...il.com>,
	Veaceslav Falico <vfalico@...il.com>,
	Andy Gospodarek <gospo@...ulusnetworks.com>,
	netdev@...r.kernel.org
Subject: Re: [RFC PATCH net] net/core: don't increment rx_dropped on inactive
 slaves

On Sat, Jan 23, 2016 at 07:23:09AM -0800, Eric Dumazet wrote:
> On Fri, 2016-01-22 at 14:11 -0500, Jarod Wilson wrote:
> 
> > ---
> >  net/core/dev.c | 3 +++
> >  1 file changed, 3 insertions(+)
> > 
> > diff --git a/net/core/dev.c b/net/core/dev.c
> > index 8cba3d8..1354c7b 100644
> > --- a/net/core/dev.c
> > +++ b/net/core/dev.c
> > @@ -4153,8 +4153,11 @@ ncls:
> >  		else
> >  			ret = pt_prev->func(skb, skb->dev, pt_prev, orig_dev);
> >  	} else {
> > +		if (deliver_exact)
> > +			goto inactive; /* bond or team inactive slave */
> >  drop:
> >  		atomic_long_inc(&skb->dev->rx_dropped);
> > +inactive:
> >  		kfree_skb(skb);
> >  		/* Jamal, now you will not able to escape explaining
> >  		 * me how you were going to use this. :-)
> 
> Note that if you still have a kfree_skb() instead of consume_skb(),
> some tools will still give you a wrong signal (packet dropped ...).
> 
> But then maybe the signal is telling some truth.
> 
> We receive a packet, and decide to drop it because no one was willing to
> handle it.
> 
> Maybe someone wants to know a particular slave receives 10,000 such
> frames per second and hurts performance with useless work.
> 
> We should at least increment some counter and maybe dump it with
> "ethtool -S" or something.

I've been digging into ethtool -S a little bit, and am somewhat at a loss
as to how I would wire into this. From what I've been able to figure out,
it's entirely device-specific-ish counters spit out. On my sfc cards, I
get rx_noskb_drops and rx_nodesc_drop_cnt output from ethtool -S, but for
the core network stack, these are actually added up and shoved into
rx_dropped, and no other network driver has those two individual counters.

By itself, rx_dropped isn't output directly anywhere from ethtool, 
so far as I can see. And ethtool -S bondX shows absolutely nothing.
*Should* ethtool -S be dumping all the network core stats? I have to say I
was more than a little surprised at this:

# ethtool -S bond0
no stats available

Particularly given that if I look in /proc/net/dev or
/sys/devices/virtual/net/bond0/statistics/*, there are quite a few stats
that are being tracked and make their way out to userspace...

-- 
Jarod Wilson
jarod@...hat.com