linux-kernel - Re: [PATCH] cpumask: alloc_cpumask_var() use NUMA_NO

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Date:	Mon,  9 May 2011 09:29:06 +0900 (JST)
From:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	kosaki.motohiro@...fujitsu.com,
	LKML <linux-kernel@...r.kernel.org>, linux-mm@...ck.org
Subject: Re: [PATCH] cpumask: alloc_cpumask_var() use NUMA_NO_NODE

Hi

> On Thu, 28 Apr 2011 23:17:15 +0900 (JST)
> KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com> wrote:
> 
> > NUMA_NO_NODE and numa_node_id() are different meanings. NUMA_NO_NODE 
> > is obviously recomended fallback.
> > 
> > Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
> > ---
> >  lib/cpumask.c |    2 +-
> >  1 files changed, 1 insertions(+), 1 deletions(-)
> > 
> > diff --git a/lib/cpumask.c b/lib/cpumask.c
> > index 4f6425d..af3e581 100644
> > --- a/lib/cpumask.c
> > +++ b/lib/cpumask.c
> > @@ -131,7 +131,7 @@ EXPORT_SYMBOL(zalloc_cpumask_var_node);
> >   */
> >  bool alloc_cpumask_var(cpumask_var_t *mask, gfp_t flags)
> >  {
> > -	return alloc_cpumask_var_node(mask, flags, numa_node_id());
> > +	return alloc_cpumask_var_node(mask, flags, NUMA_NO_NODE);
> >  }
> >  EXPORT_SYMBOL(alloc_cpumask_var);
> >  
> 
> So effectively this will replace numa_node_id() with numa_mem_id(),
> yes?  What runtime effects might this have?  

If I understand correctly,

 alloc_pages_node():  same effect both NUMA_NO_NODE and numa_node_id()
 kmalloc_node(): not same effect NUMA_NO_NODE and numa_node_id()

because 

slub.c
---------------------------------------------------
	slab_alloc() {
	 (snip)
	        if (unlikely(!object || !node_match(c, node)))
			// slow path
	        else {
			// fast path

and

	static inline int node_match(struct kmem_cache_cpu *c, int node)
	{
	#ifdef CONFIG_NUMA
	        if (node != NUMA_NO_NODE && c->node != node)
	                return 0;
	#endif
	        return 1;
	}
---------------------------------------------------
In a nutshell, numa_node_id() reduce slab cache reusing chance.


Oh, I missed slab.c code. It's using numa_mem_id(). thank you correct me!

numa_mem_id() don't have much meanings. It's ia64 HP big machine quirk.
it only affect to improve slab performance a little if users try to allocate
a memory from a cpu within memoryless node . and, 99% users never use such machine.

In a nutshell, NUMA_NO_NODE and numa_node_id() don't have a lot of difference
if users are using SLAB.


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