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

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Date:   Tue, 26 Feb 2019 13:40:15 +0800
From:   Pingfan Liu <kernelfans@...il.com>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     x86@...nel.org, linux-mm@...ck.org,
        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>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/6] x86/numa: define numa_init_array() conditional on CONFIG_NUMA

On Mon, Feb 25, 2019 at 11:24 PM Dave Hansen <dave.hansen@...el.com> wrote:
>
> 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*.

I had thought separating [3/6] and [4/6] can ease the review. And I
will merge them in next version.

Thanks and regards,
Pingfan