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Date: Thu, 16 May 2019 17:26:09 -0700
From: Kees Cook <keescook@...omium.org>
To: Alexander Potapenko <glider@...gle.com>
Cc: akpm@...ux-foundation.org, cl@...ux.com,
kernel-hardening@...ts.openwall.com,
Masahiro Yamada <yamada.masahiro@...ionext.com>,
James Morris <jmorris@...ei.org>,
"Serge E. Hallyn" <serge@...lyn.com>,
Nick Desaulniers <ndesaulniers@...gle.com>,
Kostya Serebryany <kcc@...gle.com>,
Dmitry Vyukov <dvyukov@...gle.com>,
Sandeep Patil <sspatil@...roid.com>,
Laura Abbott <labbott@...hat.com>,
Randy Dunlap <rdunlap@...radead.org>, Jann Horn <jannh@...gle.com>,
Mark Rutland <mark.rutland@....com>, linux-mm@...ck.org,
linux-security-module@...r.kernel.org
Subject: Re: [PATCH v2 4/4] net: apply __GFP_NO_AUTOINIT to AF_UNIX sk_buff
allocations
On Thu, May 16, 2019 at 09:53:01AM -0700, Kees Cook wrote:
> On Tue, May 14, 2019 at 04:35:37PM +0200, Alexander Potapenko wrote:
> > Add sock_alloc_send_pskb_noinit(), which is similar to
> > sock_alloc_send_pskb(), but allocates with __GFP_NO_AUTOINIT.
> > This helps reduce the slowdown on hackbench in the init_on_alloc mode
> > from 6.84% to 3.45%.
>
> Out of curiosity, why the creation of the new function over adding a
> gfp flag argument to sock_alloc_send_pskb() and updating callers? (There
> are only 6 callers, and this change already updates 2 of those.)
>
> > Slowdown for the initialization features compared to init_on_free=0,
> > init_on_alloc=0:
> >
> > hackbench, init_on_free=1: +7.71% sys time (st.err 0.45%)
> > hackbench, init_on_alloc=1: +3.45% sys time (st.err 0.86%)
So I've run some of my own wall-clock timings of kernel builds (which
should be an pretty big "worst case" situation, and I see much smaller
performance changes:
everything off
Run times: 289.18 288.61 289.66 287.71 287.67
Min: 287.67 Max: 289.66 Mean: 288.57 Std Dev: 0.79
baseline
init_on_alloc=1
Run times: 289.72 286.95 287.87 287.34 287.35
Min: 286.95 Max: 289.72 Mean: 287.85 Std Dev: 0.98
0.25% faster (within the std dev noise)
init_on_free=1
Run times: 303.26 301.44 301.19 301.55 301.39
Min: 301.19 Max: 303.26 Mean: 301.77 Std Dev: 0.75
4.57% slower
init_on_free=1 with the PAX_MEMORY_SANITIZE slabs excluded:
Run times: 299.19 299.85 298.95 298.23 298.64
Min: 298.23 Max: 299.85 Mean: 298.97 Std Dev: 0.55
3.60% slower
So the tuning certainly improved things by 1%. My perf numbers don't
show the 24% hit you were seeing at all, though.
> In the commit log it might be worth mentioning that this is only
> changing the init_on_alloc case (in case it's not already obvious to
> folks). Perhaps there needs to be a split of __GFP_NO_AUTOINIT into
> __GFP_NO_AUTO_ALLOC_INIT and __GFP_NO_AUTO_FREE_INIT? Right now
> __GFP_NO_AUTOINIT is only checked for init_on_alloc:
I was obviously crazy here. :) GFP isn't present for free(), but a SLAB
flag works (as was done in PAX_MEMORY_SANITIZE). I'll send the patch I
used for the above timing test.
--
Kees Cook
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