Re: [GIT PULL] scheduler fixes

Pekka Enberg
2009-05-25T18:43:41Z

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