linux-kernel - Re: [PATCH for-v3.7 2/2] slub: optimize kmalloc* inlining for GFP

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Message-ID: <1350973015.8609.1444.camel@edumazet-glaptop>
Date:	Tue, 23 Oct 2012 08:16:55 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	JoonSoo Kim <js1304@...il.com>
Cc:	Christoph Lameter <cl@...ux.com>,
	Pekka Enberg <penberg@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH for-v3.7 2/2] slub: optimize kmalloc* inlining for
 GFP_DMA

On Tue, 2012-10-23 at 11:29 +0900, JoonSoo Kim wrote:
> 2012/10/22 Christoph Lameter <cl@...ux.com>:
> > On Sun, 21 Oct 2012, Joonsoo Kim wrote:
> >
> >> kmalloc() and kmalloc_node() of the SLUB isn't inlined when @flags = __GFP_DMA.
> >> This patch optimize this case,
> >> so when @flags = __GFP_DMA, it will be inlined into generic code.
> >
> > __GFP_DMA is a rarely used flag for kmalloc allocators and so far it was
> > not considered that it is worth to directly support it in the inlining
> > code.
> >
> >
> 
> Hmm... but, the SLAB already did that optimization for __GFP_DMA.
> Almost every kmalloc() is invoked with constant flags value,
> so I think that overhead from this patch may be negligible.
> With this patch, code size of vmlinux is reduced slightly.

Only because you asked a allyesconfig

GFP_DMA is used for less than 0.1 % of kmalloc() calls, for legacy
hardware (from last century)


In fact if you want to reduce even more your vmlinux, you could test

if (__builtin_constant_p(flags) && (flags & SLUB_DMA))
    return kmem_cache_alloc_trace(s, flags, size);

to force the call to out of line code.




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