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Message-ID: <20150514181707.GA21728@gmail.com>
Date: Thu, 14 May 2015 20:17:07 +0200
From: Ingo Molnar <mingo@...nel.org>
To: Denys Vlasenko <dvlasenk@...hat.com>
Cc: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
Linus Torvalds <torvalds@...ux-foundation.org>,
Jason Low <jason.low2@...com>,
Peter Zijlstra <peterz@...radead.org>,
Davidlohr Bueso <dave@...olabs.net>,
Tim Chen <tim.c.chen@...ux.intel.com>,
Aswin Chandramouleeswaran <aswin@...com>,
LKML <linux-kernel@...r.kernel.org>,
Borislav Petkov <bp@...en8.de>,
Andy Lutomirski <luto@...capital.net>,
Brian Gerst <brgerst@...il.com>,
"H. Peter Anvin" <hpa@...or.com>,
Thomas Gleixner <tglx@...utronix.de>,
Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [PATCH] x86: Align jump targets to 1 byte boundaries
* Denys Vlasenko <dvlasenk@...hat.com> wrote:
> > What do you guys think about this? I think we should seriously
> > consider relaxing our alignment defaults.
>
> Looks like nobody objected. I think it's ok to submit
> this patch for real.
Yeah, so my plan is to apply the following three changes from that
discussion:
--- tip.orig/arch/x86/Makefile
+++ tip/arch/x86/Makefile
@@ -77,6 +77,15 @@ else
KBUILD_AFLAGS += -m64
KBUILD_CFLAGS += -m64
+ # Pack jump targets tightly, don't align them to the default 16 bytes:
+ KBUILD_CFLAGS += -falign-jumps=1
+
+ # Pack functions tightly as well:
+ KBUILD_CFLAGS += -falign-functions=1
+
+ # Pack loops tightly as well:
+ KBUILD_CFLAGS += -falign-loops=1
+
# Don't autogenerate traditional x87 instructions
KBUILD_CFLAGS += $(call cc-option,-mno-80387)
KBUILD_CFLAGS += $(call cc-option,-mno-fp-ret-in-387)
... and not do -fno-guess-branch-probability, because it destroys
likely()/unlikely() annotations.
Which is a pity, considering the size effect on defconfig:
text data bss dec filename
12566383 1617840 1089536 15273759 vmlinux.expect=10 [==vanilla]
11923529 1617840 1089536 14630905 vmlinux.-fno-guess-branch-probability
11903663 1617840 1089536 14611039 vmlinux.align=1
11646102 1617840 1089536 14353478 vmlinux.align=1+fno-guess-branch-probability
I.e. 2.6% of savings on top of the above three patches, while the
effect of our hot/cold branch annotations is only around 0.4%, so if
GCC preserved our annotations under -fno-guess-branch-probability we'd
be good by at least 2%.
But GCC doesn't.
There were also these other changes I tested:
+ # Reduces vmlinux size by 0.25%:
+ KBUILD_CFLAGS += -fno-caller-saves
+
+ # Reduces vmlinux size by 1.10%:
+ KBUILD_CFLAGS += -fno-inline-small-functions
+
+ # Reduces vmlinux size by about 0.95%:
+ KBUILD_CFLAGS += -fno-tree-ch
We could maybe consider -fno-caller-saves. What do you think about
that option?
-fno-inline-small-functions is probably a bad idea, and -fno-tree-ch
is probably a bad idea as well and is a dangerously rare option in any
case that could break in unexpected ways.
Thanks,
Ingo
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