linux-kernel - Re: [GIT PULL] scheduler fixes

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Date:	Mon, 25 May 2009 09:39:36 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Pekka J Enberg <penberg@...helsinki.fi>
cc:	Nick Piggin <npiggin@...e.de>, Ingo Molnar <mingo@...e.hu>,
	Yinghai Lu <yinghai@...nel.org>,
	Rusty Russell <rusty@...tcorp.com.au>,
	"H. Peter Anvin" <hpa@...or.com>, Jeff Garzik <jgarzik@...ox.com>,
	Alexander Viro <viro@....linux.org.uk>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	cl@...ux-foundation.org, mpm@...enic.com
Subject: Re: [GIT PULL] scheduler fixes

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().

		Linus
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