Re: [patch 03/11] mm, mempolicy: remove per-process flag

[Andrew Morton]

On Fri, 07 Mar 2014 09:20:39 -0800 Andi Kleen <andi@xxxxxxxxxxxxxx> wrote:

> David Rientjes <rientjes@xxxxxxxxxx> writes:
> >
> > Per-process flags are a scarce resource so we should free them up
> > whenever possible and make them available.  We'll be using it shortly for
> > memcg oom reserves.
> 
> I'm not convinced TCP_RR is a meaningfull benchmark for slab.
> 
> The shortness seems like an artificial problem.
> 
> Just add another flag word to the task_struct? That would seem 
> to be the obvious way. People will need it sooner or later anyways.
> 

This is basically what the patch does:

@@ -3259,7 +3259,7 @@ __do_cache_alloc(struct kmem_cache *cach
 {
 	void *objp;
 
-	if (unlikely(current->flags & (PF_SPREAD_SLAB | PF_MEMPOLICY))) {
+	if (current->mempolicy || unlikely(current->flags & PF_SPREAD_SLAB)) {
 		objp = alternate_node_alloc(cache, flags);
 		if (objp)
 			goto out;

It runs when slab goes into the page allocator for backing store (ie:
relatively rarely).  It adds one test-n-branch when a mempolicy is
active and actually removes instructions when no mempolicy is active.

This patch won't be making any difference to anything.
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