linux-kernel - Re: [PATCH 4/9] net: openvswitch: use this_cpu

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Date:	Wed, 31 Oct 2012 17:39:54 +0000
From:	Christoph Lameter <cl@...ux.com>
To:	Shan Wei <shanwei88@...il.com>
cc:	jesse@...ira.com, dev@...nvswitch.org,
	NetDev <netdev@...r.kernel.org>,
	Kernel-Maillist <linux-kernel@...r.kernel.org>,
	David Miller <davem@...emloft.net>
Subject: Re: [PATCH 4/9] net: openvswitch: use this_cpu_ptr per-cpu helper

On Wed, 31 Oct 2012, Shan Wei wrote:

> --- a/net/openvswitch/datapath.c
> +++ b/net/openvswitch/datapath.c
> @@ -208,7 +208,7 @@ void ovs_dp_process_received_packet(struct vport *p, struct sk_buff *skb)
>  	int error;
>  	int key_len;
>
> -	stats = per_cpu_ptr(dp->stats_percpu, smp_processor_id());
> +	stats = this_cpu_ptr(dp->stats_percpu);

Well this is an improvement and may be ok if the preemption is disabled at
this point. There is another possibility here to use this_cpu_read/add/inc
instead of determining the pointer to the local cpu first and then
performing operations on the fields. The pointer relocation with
this_cpu_xxx ops is implicit in the instructions and safe against changing
of processors. It would also save us the determination of a pointer to the
current cpus stats structure.

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