netdev - Re: [PATCH net-next] udp: intoduce udp_encap_needed static

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Date:	Thu, 12 Apr 2012 11:10:11 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Simon Horman <horms@...ge.net.au>
Cc:	David Miller <davem@...emloft.net>, dev@...nvswitch.org,
	netdev@...r.kernel.org
Subject: Re: [PATCH net-next] udp: intoduce udp_encap_needed static_key

On Thu, 2012-04-12 at 11:05 +0200, Eric Dumazet wrote:

> If static_key is not yet enabled, the fast path does a single JMP .
> 
> When static_key is enabled, JMP destination is patched to reach the real
> encap_type/encap_rcv logic, possibly adding cache misses.

Small note Simon,

The jump trick is effective on x86 (and maybe some other arches) when

CONFIG_JUMP_LABEL=y

Else, its replaced by atomic_read(...) > 0, a cnditional jump but
reading a read_mostly/shared variable, instead of a per socket field.




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