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

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Date:	Mon, 25 May 2009 21:39:08 +0300
From:	Pekka Enberg <penberg@...helsinki.fi>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
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

Hi Linus,

Linus Torvalds wrote:
> 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).

I can certainly fix that up to use kmalloc() or vmalloc(). I moved it 
because I wasn't sure how much it's actually allocating and wanted to do 
the conservative thing here.

Linus Torvalds wrote:
> 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".

SLUB does sysfs setup in kmem_cache_init() and if I saw some oopses if I 
don't call vfs_caches_init_early() first. I didn't look too closely, though.

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

We can do that later but then we need to fix up vmalloc_init(). There's 
actually a patch floating around to do that.

			Pekka
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