linux-kernel - Re: [PATCH] x86_64: remove never used nodenumer in pda

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Date:	Mon, 18 Feb 2008 12:16:14 -0800
From:	"Yinghai Lu" <yhlu.kernel@...il.com>
To:	"Andi Kleen" <andi@...stfloor.org>
Cc:	"Ingo Molnar" <mingo@...e.hu>,
	"Andrew Morton" <akpm@...ux-foundation.org>,
	"Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>,
	"Mike Travis" <travis@....com>, lameter@....com
Subject: Re: [PATCH] x86_64: remove never used nodenumer in pda

On Feb 18, 2008 4:23 AM, Andi Kleen <andi@...stfloor.org> wrote:
> Yinghai Lu <Yinghai.Lu@....COM> writes:
>
> > we don't need copy too. already have x86_cpu_to_node_map
>
> That's a regression (probably from Mike's patches?). Until recently it was
> used.
>
> The reason the node number was put in there is that it generates far
> shorter code to just fetch the local node number from the PDA than to
> first go through a array lookup from the cpu number. It also saves a
> costly cache line miss on the array if you're unlucky.
>
> It is far better to fix it than to remove it.
>
> I know Mike/Christoph want to get rid of the PDA and make per cpu data
> as efficient as the PDA. If that happens the right fix is to create
> a new per CPU data variable for the node number again.
>
> Here's a quick patch (tested on kvm with numa emulation only)
>
> It should be ok because PDA is set up early and
> the early node is always 0 and there is a 0 in there
> at early boot.
>
> Saves about 1.6k of text on a vmlinux here.

that is because of the inline...

static inline int early_cpu_to_node(int cpu)
{
        int *cpu_to_node_map = x86_cpu_to_node_map_early_ptr;

        if (cpu_to_node_map)
                return cpu_to_node_map[cpu];
        else if (per_cpu_offset(cpu))
                return per_cpu(x86_cpu_to_node_map, cpu);
        else
                return NUMA_NO_NODE;
}

should use per_cpu(x86_cpu_to_node_map, cpu) instead.
and limited using early_cpu_to_node etc.

YH
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