Re: [PATCH 05/13] mm: meminit: Make __early_pfn_to_nid SMP-safe and introduce meminit_pfn_in_nid

From: Andrew Morton
Date: Mon Apr 27 2015 - 18:43:40 EST

On Thu, 23 Apr 2015 11:33:08 +0100 Mel Gorman <mgorman@xxxxxxx> wrote:

> __early_pfn_to_nid() in the generic and arch-specific implementations
> use static variables to cache recent lookups. Without the cache
> boot times are much higher due to the excessive memblock lookups but
> it assumes that memory initialisation is single-threaded. Parallel
> initialisation of struct pages will break that assumption so this patch
> makes __early_pfn_to_nid() SMP-safe by requiring the caller to cache
> recent search information. early_pfn_to_nid() keeps the same interface
> but is only safe to use early in boot due to the use of a global static
> variable. meminit_pfn_in_nid() is an SMP-safe version that callers must
> maintain their own state for.

Seems a bit awkward.

> +struct __meminitdata mminit_pfnnid_cache global_init_state;
> +
> +/* Only safe to use early in boot when initialisation is single-threaded */
> int __meminit early_pfn_to_nid(unsigned long pfn)
> {
> int nid;
> - nid = __early_pfn_to_nid(pfn);
> + /* The system will behave unpredictably otherwise */
> + BUG_ON(system_state != SYSTEM_BOOTING);

Because of this.

Providing a cache per cpu:

struct __meminitdata mminit_pfnnid_cache global_init_state[NR_CPUS];

would be simpler?

Also, `global_init_state' is a poor name for a kernel-wide symbol.

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