From: Nicholas Piggin <npiggin@gmail.com>
To: Nicolas Pitre <nicolas.pitre@linaro.org>
Cc: linux-kbuild@vger.kernel.org, linux-arch@vger.kernel.org,
linuxppc-dev@lists.ozlabs.org,
Stephen Rothwell <sfr@canb.auug.org.au>,
Arnd Bergmann <arnd@arndb.de>,
Segher Boessenkool <segher@kernel.crashing.org>,
Alan Modra <amodra@gmail.com>
Subject: Re: [PATCH 2/5] kbuild: allow archs to select build for link dead code/data elimination
Date: Mon, 8 Aug 2016 14:27:42 +1000 [thread overview]
Message-ID: <20160808142742.52bb4b1e@roar.ozlabs.ibm.com> (raw)
In-Reply-To: <alpine.LFD.2.20.1608080004050.17623@knanqh.ubzr>
On Mon, 8 Aug 2016 00:12:37 -0400 (EDT)
Nicolas Pitre <nicolas.pitre@linaro.org> wrote:
> On Mon, 8 Aug 2016, Nicholas Piggin wrote:
>
> > On Sun, 7 Aug 2016 01:33:45 -0400 (EDT)
> > Nicolas Pitre <nicolas.pitre@linaro.org> wrote:
> >
> > > On Fri, 5 Aug 2016, Nicholas Piggin wrote:
> > >
> > > > Introduce LINKER_DCE option for architectures to select if they want
> > > > to build with -ffunction-sections, -fdata-sections, and link with
> > > > --gc-sections. It requires some work (documented) to ensure all
> > > > unreferenced entrypoints are live, and requires toolchain and
> > > > build verification, so it is made a per-arch option for now.
> > > >
> > > > On a random powerpc64le build, this yelds a significant size saving,
> > > > it boots and runs fine, but there is a lot I haven't tested as yet,
> > > > so these savings may be reduced if there are bugs in the link.
> > > >
> > > > text data bss dec filename
> > > > 11169741 1180744 1923176 14273661 vmlinux
> > > > 10445269 1004127 1919707 13369103 vmlinux.dce
> > > >
> > > > ~700K text, ~170K data, 6% removed from kernel image size.
> > > >
> > > > Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
> > >
> > > I played with that too. However this needs distinct sections for
> > > exception tables and the like otherwise the backward references from the
> > > final exception table to those functions responsible for those exception
> > > entries has the effect of pulling in all those functions even if their
> > > entry point is never referenced, making --gc-sections less effective.
> > > I managed to fix this only with a change to gas (accepted upstream).
> > >
> > > But once that is solved, you then have the missing forward reference
> > > problem i.e. nothing actually references those individual exception
> > > entry sections and ld happily drops them all. Having a KEEP() on each of
> > > them is unworkable and defeats the purpose anyway. That requires a
> > > dummy reloc to trick ld into pulling in those sections when the parent
> > > section is also pulled in.
> >
> > Right, although we don't *need* those things just for enabling
> > --gc-sections, do we? It may not be 100% optimal, but it's enough
> > to avoid the regression when switching to --whole-archive build
> > option.
>
> Oh absolutely.
>
> > Your results are impressive, and I don't want to stand in the way of
> > either LTO or improving accuracy of --gc-sections. But both are things
> > that can be built on top of this patch, I think.
>
> Indeed. Those patches are certainly welcome. They represent half of the
> job already. I just wanted to provide some insight about the whole
> picture in case someone else notices those flaws I have identified.
Okay thanks, I appreciate you taking a look. I wanted to be sure I
wasn't missing some bug here.
Smaller kernel is nice for large systems because it means smaller
icache/dcache footprint and fewer branch trampolines, so I'm always
happy to see that effort. I will certainly help test LTO or some of
these other gc-sections improvements on powerpc.
Thanks,
Nick
next prev parent reply other threads:[~2016-08-08 4:27 UTC|newest]
Thread overview: 40+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-08-05 12:11 [RFC][PATCH 0/5] kbuild changes, thin archives, --gc-sections Nicholas Piggin
2016-08-05 12:11 ` [PATCH 1/5] kbuild: allow architectures to use thin archives instead of ld -r Nicholas Piggin
2016-08-06 3:50 ` kbuild test robot
2016-08-06 20:10 ` Sam Ravnborg
2016-08-07 1:49 ` Stephen Rothwell
2016-08-07 3:34 ` Alan Modra
2016-08-07 4:17 ` Nicolas Pitre
2016-08-07 14:40 ` Sam Ravnborg
2016-08-08 3:19 ` Nicholas Piggin
2016-08-08 4:46 ` Sam Ravnborg
2016-08-08 3:25 ` Nicholas Piggin
2016-08-08 9:18 ` Arnd Bergmann
2016-08-05 12:12 ` [PATCH 2/5] kbuild: allow archs to select build for link dead code/data elimination Nicholas Piggin
2016-08-06 20:14 ` Sam Ravnborg
2016-08-08 3:29 ` Nicholas Piggin
2016-08-08 4:49 ` Sam Ravnborg
2016-08-07 5:33 ` Nicolas Pitre
2016-08-08 3:42 ` Nicholas Piggin
2016-08-08 4:12 ` Nicolas Pitre
2016-08-08 4:27 ` Nicholas Piggin [this message]
2016-08-07 9:57 ` Alan Modra
2016-08-07 11:35 ` Andreas Schwab
2016-08-07 20:26 ` Arnd Bergmann
2016-08-07 23:49 ` Alan Modra
2016-08-08 15:14 ` Arnd Bergmann
2016-08-08 23:50 ` Alan Modra
2016-08-09 22:10 ` Arnd Bergmann
2016-08-09 3:16 ` Andi Kleen
2016-08-09 22:29 ` Arnd Bergmann
2016-08-09 23:08 ` Andi Kleen
2016-08-10 0:37 ` Andi Kleen
2016-08-05 12:12 ` [PATCH 3/5] kbuild: add arch specific post-module-link pass Nicholas Piggin
2016-08-05 13:56 ` Nicholas Piggin
2016-08-06 20:16 ` Sam Ravnborg
2016-08-08 3:30 ` Nicholas Piggin
2016-08-05 12:12 ` [PATCH 4/5] powerpc: switch to using thin archives Nicholas Piggin
2016-08-05 12:12 ` [PATCH 5/5] powerpc/64: use linker dce Nicholas Piggin
2016-08-05 13:32 ` [RFC][PATCH 0/5] kbuild changes, thin archives, --gc-sections Nicholas Piggin
2016-08-07 20:23 ` Arnd Bergmann
2016-08-08 3:53 ` Nicholas Piggin
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