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

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Message-Id: <20140307124831.69b50f829ed34de8651fa461@linux-foundation.org>
Date:	Fri, 7 Mar 2014 12:48:31 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	David Rientjes <rientjes@...gle.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Michal Hocko <mhocko@...e.cz>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Pekka Enberg <penberg@...nel.org>, Tejun Heo <tj@...nel.org>,
	Mel Gorman <mgorman@...e.de>, Oleg Nesterov <oleg@...hat.com>,
	Rik van Riel <riel@...hat.com>,
	Jianguo Wu <wujianguo@...wei.com>,
	Tim Hockin <thockin@...gle.com>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, cgroups@...r.kernel.org,
	linux-doc@...r.kernel.org
Subject: Re: [patch 03/11] mm, mempolicy: remove per-process flag

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

> David Rientjes <rientjes@...gle.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.
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