netdev - Re: irq disable in __netdev_alloc

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Date:	Wed, 22 Oct 2014 18:52:40 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Alexei Starovoitov <ast@...mgrid.com>
Cc:	Eric Dumazet <edumazet@...gle.com>,
	Network Development <netdev@...r.kernel.org>
Subject: Re: irq disable in __netdev_alloc_frag() ?

On Wed, 2014-10-22 at 17:15 -0700, Alexei Starovoitov wrote:
> Hi Eric,
> 
> in the commit 6f532612cc24 ("net: introduce netdev_alloc_frag()")
> you mentioned that the reason to disable interrupts
> in __netdev_alloc_frag() is:
> "- Must be IRQ safe (non NAPI drivers can use it)"
> 
> Is there a way to do this conditionally?
> 
> Without it I see 10% performance gain for my RX tests
> (from 6.9Mpps to 7.7Mpps) and __netdev_alloc_frag()
> itself goes from 6.6% to 2.1%
> (popf seems to be quite costly)

Well, your driver is probably a NAPI one, so you need to
mask irqs, or to remove all non NAPI drivers from linux.

__netdev_alloc_frag() (__netdev_alloc_skb()) is used by all.

Problem is __netdev_alloc_frag() is generally deep inside caller
chain, so using a private pool might have quite an overhead.

Same could be said for skb_queue_head() /skb_queue_tail() /
sock_queue_rcv_skb() :
Many callers don't need to block irq.

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