linux-kernel - Re: [PATCH] arm64: Increase the max granular size

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Message-ID: <20151104153910.GN7637@e104818-lin.cambridge.arm.com>
Date:	Wed, 4 Nov 2015 15:39:10 +0000
From:	Catalin Marinas <catalin.marinas@....com>
To:	Christoph Lameter <cl@...ux.com>
Cc:	Robert Richter <rric@...nel.org>, Joonsoo Kim <js1304@...il.com>,
	Linux-sh list <linux-sh@...r.kernel.org>,
	Will Deacon <will.deacon@....com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Robert Richter <rrichter@...ium.com>, linux-mm@...ck.org,
	Tirumalesh Chalamarla <tchalamarla@...ium.com>,
	Geert Uytterhoeven <geert@...ux-m68k.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH] arm64: Increase the max granular size

On Wed, Nov 04, 2015 at 09:28:34AM -0600, Christoph Lameter wrote:
> On Wed, 4 Nov 2015, Catalin Marinas wrote:
> 
> > BTW, assuming L1_CACHE_BYTES is 512 (I don't ever see this happening but
> > just in theory), we potentially have the same issue. What would save us
> > is that INDEX_NODE would match the first "kmalloc-512" cache, so we have
> > it pre-populated.
> 
> Ok maybe add some BUILD_BUG_ONs to ensure that builds fail until we have
> addressed that.

A BUILD_BUG_ON should be fine.

Thinking some more, I think if KMALLOC_MIN_SIZE is 128, there is no gain
with off-slab management since the freelist allocation would still be
128 bytes. An alternative to reverting while still having a little
benefit of off-slab for 256 bytes objects (rather than 512 as we would
get with the revert):

diff --git a/mm/slab.c b/mm/slab.c
index 4fcc5dd8d5a6..ac32b4a0f2ec 100644
--- a/mm/slab.c
+++ b/mm/slab.c
@@ -2212,8 +2212,8 @@ __kmem_cache_create (struct kmem_cache *cachep, unsigned long flags)
 	 * it too early on. Always use on-slab management when
 	 * SLAB_NOLEAKTRACE to avoid recursive calls into kmemleak)
 	 */
-	if ((size >= (PAGE_SIZE >> 5)) && !slab_early_init &&
-	    !(flags & SLAB_NOLEAKTRACE))
+	if ((size >= (PAGE_SIZE >> 5)) && (size > KMALLOC_MIN_SIZE) &&
+		!slab_early_init && !(flags & SLAB_NOLEAKTRACE))
 		/*
 		 * Size is large, assume best to place the slab management obj
 		 * off-slab (should allow better packing of objs).

Whichever you prefer.

-- 
Catalin
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