On Mon, Apr 27, 2015 at 03:43:33PM -0700, Andrew Morton wrote:
> 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.
>
I'm afraid I don't understand which part you mean.
> > +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?
>
It would be simplier in terms of implementation but it's wasteful. We
only need a small number of these caches early in boot. NR_CPUS is
potentially very large.
>
> Also, `global_init_state' is a poor name for a kernel-wide symbol.
You're right. It's not really global, it's just the one that is used if
the caller does not track their own state. It should have been static and
I renamed it to early_pfnnid_cache.
--
Mel Gorman
SUSE Labs
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