From mboxrd@z Thu Jan  1 00:00:00 1970
From: Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [patch 03/11] mm, mempolicy: remove per-process flag
Date: Fri, 7 Mar 2014 12:48:31 -0800
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To: Andi Kleen <andi@firstfloor.org>
Cc: David Rientjes <rientjes@google.com>, Johannes Weiner <hannes@cmpxchg.org>, Michal Hocko <mhocko@suse.cz>, KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>, Christoph Lameter <cl@linux-foundation.org>, Pekka Enberg <penberg@kernel.org>, Tejun Heo <tj@kernel.org>, Mel Gorman <mgorman@suse.de>, Oleg Nesterov <oleg@redhat.com>, Rik van Riel <riel@redhat.com>, Jianguo Wu <wujianguo@huawei.com>, Tim Hockin <thockin@google.com>, linux-kernel@vger.kernel.org, linux-mm@kvack.org, cgroups@vger.kernel.org, linux-doc@vger.kernel.org

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

> David Rientjes <rientjes@google.com> 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.