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Date: Fri, 20 May 2016 11:15:58 +0900
Message-ID: <CAAmzW4PN4wcPWbjf=Hws2qN_eZC1HCmn-gQC9_DB5ek5+bNksQ@mail.gmail.com>
Subject: Re: [RFC v1 2/2] mm: SLUB Freelist randomization
From: Joonsoo Kim <js1304@gmail.com>
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To: Thomas Garnier <thgarnie@google.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>, Christoph Lameter <cl@linux.com>, Pekka Enberg <penberg@kernel.org>, David Rientjes <rientjes@google.com>, Andrew Morton <akpm@linux-foundation.org>, "Paul E . McKenney" <paulmck@linux.vnet.ibm.com>, Pranith Kumar <bobby.prani@gmail.com>, David Howells <dhowells@redhat.com>, Tejun Heo <tj@kernel.org>, Johannes Weiner <hannes@cmpxchg.org>, David Woodhouse <David.Woodhouse@intel.com>, Petr Mladek <pmladek@suse.com>, Kees Cook <keescook@chromium.org>, Linux-MM <linux-mm@kvack.org>, LKML <linux-kernel@vger.kernel.org>, Greg Thelen <gthelen@google.com>, kernel-hardening@lists.openwall.com

2016-05-20 5:20 GMT+09:00 Thomas Garnier <thgarnie@google.com>:
> I ran the test given by Joonsoo and it gave me these minimum cycles
> per size across 20 usage:

I can't understand what you did here. Maybe, it's due to my poor Engling.
Please explain more. You did single thread test? Why minimum cycles
rather than average?

> size,before,after
> 8,63.00,64.50 (102.38%)
> 16,64.50,65.00 (100.78%)
> 32,65.00,65.00 (100.00%)
> 64,66.00,65.00 (98.48%)
> 128,66.00,65.00 (98.48%)
> 256,64.00,64.00 (100.00%)
> 512,65.00,66.00 (101.54%)
> 1024,68.00,64.00 (94.12%)
> 2048,66.00,65.00 (98.48%)
> 4096,66.00,66.00 (100.00%)

It looks like performance of all size classes are the same?

> I assume the difference is bigger if you don't have RDRAND support.

What does RDRAND means? Kconfig? How can I check if I have RDRAND?

> Christoph, Joonsoo: Do you think it would be valuable to add a CONFIG
> to disable additional randomization per new page? It will remove
> additional entropy but increase performance for machines without arch
> specific randomization instructions.

I don't think that it deserve another CONFIG. If performance is a matter,
I think that removing additional entropy is better until it is proved that
entropy is a problem.

Thanks.

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