netdev - Re: [PATCH] net: Update netdev_alloc_frag to work more efficiently with TCP and GRO

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Date:	Wed, 20 Jun 2012 22:56:53 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	eric.dumazet@...il.com
Cc:	alexander.h.duyck@...el.com, netdev@...r.kernel.org,
	jeffrey.t.kirsher@...el.com
Subject: Re: [PATCH] net: Update netdev_alloc_frag to work more efficiently
 with TCP and GRO

From: Eric Dumazet <eric.dumazet@...il.com>
Date: Wed, 20 Jun 2012 10:17:03 +0200

> By the way, big cost in netdev_alloc_frag() is the irq
> masking/restore We probably could have a version for softirq
> users...

That's an extremely disappointing way for us to be losing cycles.

Everyone pays this price purely because:

1) __netdev_alloc_skb() uses netdev_alloc_frag()

and:

2) #1 is invoked, either directly or indirectly, by tons
   of slow non-NAPI drivers.

This got me looking into the plathora of interfaces we let drivers use
to allocate RX buffers.  It's a big mess.

We have dev_alloc_skb() which essentially calls __netdev_alloc_skb()
with a NULL device argument.  This is terrible because it means that
if we want to do something interesting on a per-device level we can't
rely upon the device being non-NULL in __netdev_alloc_skb().

I looked at the remaining dev_alloc_skb() users and these are in places
which are allocating packets in a module which is one level removed from
the netdevice level.  For example, ATM and infiniband IPATH.

What these callers want is something more like:

static inline struct sk_buff *alloc_skb_and_reserve_pad(unsinged int length,
							gfp_t gfp_mask)
{
	struct sk_buff *skb = __alloc_skb(length + NET_SKB_PAD, gfp_mask,
					  0, NUMA_NO_NODE);
	if (likely(skb))
		skb_reserve(skb, NET_SKB_PAD);
	return skb;
}

Then we won't have the NULL device case for __netdev_alloc_skb() any more.
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