Re: [RFC PATCH net] net/core: don't increment rx_dropped on inactive slaves

[Andy Gospodarek <gospo@cumulusnetworks.com>]

On Fri, Jan 22, 2016 at 02:11:22PM -0500, Jarod Wilson wrote:
> The network core tries to keep track of dropped packets, but some packets
> you wouldn't really call dropped, so much as intentionally ignored, under
> certain circumstances. One such case is that of bonding and team device
> slaves that are currently inactive. Their respective rx_handler functions
> return RX_HANDLER_EXACT (the only places in the kernel that return that),
> which ends up tracking into the network core's __netif_receive_skb_core()
> function's drop path, with no pt_prev set. On a noisy network, this can
> result in a very rapidly incrementing rx_dropped counter, not only on the
> inactive slave(s), but also on the master device, such as the following:
> 
> Inter-|   Receive                                                |  Transmit
>  face |bytes    packets errs drop fifo frame compressed multicast|bytes    packets errs drop fifo colls carrier compressed
>   p7p1: 14783346  140430    0 140428    0     0          0      2040      680       8    0    0    0     0       0          0
>   p7p2: 14805198  140648    0    0    0     0          0      2034        0       0    0    0    0     0       0          0
>  bond0: 53365248  532798    0 421160    0     0          0    115151     2040      24    0    0    0     0       0          0
>     lo:    5420      54    0    0    0     0          0         0     5420      54    0    0    0     0       0          0
>   p5p1: 19292195  196197    0 140368    0     0          0     56564      680       8    0    0    0     0       0          0
>   p5p2: 19289707  196171    0 140364    0     0          0     56547      680       8    0    0    0     0       0          0
>    em3: 20996626  158214    0    0    0     0          0       383        0       0    0    0    0     0       0          0
>    em2: 14065122  138462    0    0    0     0          0       310        0       0    0    0    0     0       0          0
>    em1: 14063162  138440    0    0    0     0          0       308        0       0    0    0    0     0       0          0
>    em4: 21050830  158729    0    0    0     0          0       385    71662     469    0    0    0     0       0          0
>    ib0:       0       0    0    0    0     0          0         0        0       0    0    0    0     0       0          0
> 
> In this scenario, p5p1, p5p2 and p7p1 are all inactive slaves in an
> active-backup bond0, and you can see that all three have high drop counts,
> with the master bond0 showing a tally of all three.
> 
> I know that this was previously discussed some here:
> 
>     http://www.spinics.net/lists/netdev/msg226341.html
> 
> It seems additional counters never came to fruition, but honestly, for
> this particular case, I'm not even sure they're warranted, I'd be inclined
> to say just silently drop these packets without incrementing a counter. At
> least, that's probably what would make someone who has complained loudly
> about this issue happy, as they have monitoring tools that are squaking
> loudly at any increments to rx_dropped.

I completely agree.

> CC: "David S. Miller" <davem@davemloft.net>
> CC: Eric Dumazet <edumazet@google.com>
> CC: Jiri Pirko <jiri@mellanox.com>
> CC: Daniel Borkmann <daniel@iogearbox.net>
> CC: Tom Herbert <tom@herbertland.com>
> CC: Jay Vosburgh <j.vosburgh@gmail.com>
> CC: Veaceslav Falico <vfalico@gmail.com>
> CC: Andy Gospodarek <gospo@cumulusnetworks.com>
> CC: netdev@vger.kernel.org
> Signed-off-by: Jarod Wilson <jarod@redhat.com>

Acked-by: Andy Gospodarek <gospo@cumulusnetworks.com>

> ---
>  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. :-)
> -- 
> 1.8.3.1
>