linux-kernel - Re: [PATCH RESEND] mm/vmscan : use vmcan_swappiness( ) basing on MEMCG config to elimiate unnecessary runtime cost

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Message-ID: <20130903224731.GC1412@cmpxchg.org>
Date:	Tue, 3 Sep 2013 18:47:31 -0400
From:	Johannes Weiner <hannes@...xchg.org>
To:	larmbr <nasa4836@...il.com>
Cc:	linux-mm@...ck.org, mhocko@...e.cz, bsingharora@...il.com,
	kamezawa.hiroyu@...fujitsu.com, akpm@...ux-foundation.org,
	mgorman@...e.de, riel@...hat.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH RESEND] mm/vmscan : use vmcan_swappiness( ) basing on
 MEMCG config to elimiate unnecessary runtime cost

On Mon, Aug 26, 2013 at 09:36:58PM +0800, larmbr wrote:
> Currently, we get the vm_swappiness via vmscan_swappiness(), which
> calls global_reclaim() to check if this is a global reclaim. 
> 
> Besides, the current implementation of global_reclaim() always returns 
> true for the !CONFIG_MEGCG case, and judges the other case by checking 
> whether scan_control->target_mem_cgroup is null or not.
> 
> Thus, we could just use two versions of vmscan_swappiness() based on 
> MEMCG Kconfig , to eliminate the unnecessary run-time cost for 
> the !CONFIG_MEMCG at all, and to squash all memcg-related checking
> into the CONFIG_MEMCG version.

The compiler can easily detect that global_reclaim() always returns
true for !CONFIG_MEMCG during compile time and not even generate a
branch for this.

> 2 files changed, 12 insertions(+), 3 deletions(-)

You're adding more code for now gain.
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