Re: [GIT PULL] scheduler fixes

From: Linus Torvalds
Date: Mon May 25 2009 - 12:42:41 EST

On Mon, 25 May 2009, Pekka J Enberg wrote:
> diff --git a/init/main.c b/init/main.c
> index 33ce929..fb0e004 100644
> --- a/init/main.c
> +++ b/init/main.c
> @@ -576,6 +576,22 @@ asmlinkage void __init start_kernel(void)
> setup_nr_cpu_ids();
> smp_prepare_boot_cpu(); /* arch-specific boot-cpu hooks */
> + build_all_zonelists();
> + page_alloc_init();
> +
> + printk(KERN_NOTICE "Kernel command line: %s\n", boot_command_line);
> + parse_early_param();
> + parse_args("Booting kernel", static_command_line, __start___param,
> + __stop___param - __start___param,
> + &unknown_bootoption);
> + pidhash_init();
> + vmalloc_init();
> + vfs_caches_init_early();
> + /*
> + * Set up kernel memory allocators
> + */
> + mem_init();
> + kmem_cache_init();

So what strikes me is a question:

- why do we want to do pidhash_init and vfs_caches_init_early() so early?

Yes, pidhash_init() now uses alloc_bootmem. It's an allocation that is not
trivially small, but it's not humongous either (max 4096 hash list heads,
one pointer each).

And vfs_caches_init_early() is actually doing some rather strange things,
like doing a "alloc_large_system_hash()" but not unconditionally: it does
it in the "late" initialization too, if not done early. inode_init_early
does soemthing very similar (ie a _conditional_ early init).

So none of this seems to really get a huge advantage from the early init.
There seems to be some subtle NUMA issues, but do we really want that? I
get the feeling that nobody ever wanted to do it early, and then the NUMA
people said "I don't wnt to do this early, but I don't want to touch the
non-NUMA case, so I'll do it early for non-numa, and late for numa".

I'm also not entirely sure we really need to do vmalloc_init() that early,
but I dunno. It also uses alloc_bootmem().

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