Re: [GIT PULL] scheduler fixes

[Nick Piggin]

On Mon, May 25, 2009 at 09:39:36AM -0700, 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).

It would be nice to use the regular page allocator for pidhash_init; For
my case, I have this patch floating around for a long time which can make
this (among other things) dynamically resizable without using locking, and
avoiding the special case for the bootmem allocated hash would be good. 


> 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".
 
vfs_caches_init_early wants to allocate with bootmem so it can get
>= MAX_ORDER cache size in the kernel direct mapping. In the NUMA
case, it is more important to spread the memory usage and utilisation
over nodes I guess so they use vmalloc for that. Bootmem and vmalloc
are not available at the same time, so it has to be 2 cases.


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

Probably not. vmalloc doesn't really work without the page allocator and
slab allocator already up, so it can probably be moved after them.

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