linux-kernel - Re: [PATCH 3/6] x86/numa: define numa_init_array() conditional on CONFIG

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Message-ID: <8f703c5f-44c7-3a96-487e-3bdf46ee41b0@intel.com>
Date:   Mon, 25 Feb 2019 07:23:59 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     Pingfan Liu <kernelfans@...il.com>, x86@...nel.org,
        linux-mm@...ck.org
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mel Gorman <mgorman@...e.de>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Andy Lutomirski <luto@...nel.org>,
        Andi Kleen <ak@...ux.intel.com>,
        Petr Tesarik <ptesarik@...e.cz>,
        Michal Hocko <mhocko@...e.com>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        Jonathan Corbet <corbet@....net>,
        Nicholas Piggin <npiggin@...il.com>,
        Daniel Vacek <neelx@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/6] x86/numa: define numa_init_array() conditional on
 CONFIG_NUMA

On 2/24/19 4:34 AM, Pingfan Liu wrote:
> +#ifdef CONFIG_NUMA
>  /*
>   * There are unfortunately some poorly designed mainboards around that
>   * only connect memory to a single CPU. This breaks the 1:1 cpu->node
> @@ -618,6 +619,9 @@ static void __init numa_init_array(void)
>  		rr = next_node_in(rr, node_online_map);
>  	}
>  }
> +#else
> +static void __init numa_init_array(void) {}
> +#endif

What functional effect does this #ifdef have?

Let's look at the code:

> static void __init numa_init_array(void)
> {
>         int rr, i;
> 
>         rr = first_node(node_online_map);
>         for (i = 0; i < nr_cpu_ids; i++) {
>                 if (early_cpu_to_node(i) != NUMA_NO_NODE)
>                         continue;
>                 numa_set_node(i, rr);
>                 rr = next_node_in(rr, node_online_map);
>         }
> }

and "play compiler" for a bit.

The first iteration will see early_cpu_to_node(i)==1 because:

static inline int early_cpu_to_node(int cpu)
{
        return 0;
}

if CONFIG_NUMA=n.

In other words, I'm not sure this patch does *anything*.