Re: powerpc allyesconfig / allmodconfig linux-next next-20160729 - next-20160729 build failures

Arnd Bergmann
2016-08-11T13:05:05Z

On Thursday, August 11, 2016 10:43:20 PM CEST Nicholas Piggin wrote:
> On Wed, 03 Aug 2016 22:13:28 +0200
> Arnd Bergmann <arnd@arndb.de> wrote:
>
> > On Wednesday, August 3, 2016 2:44:29 PM CEST Segher Boessenkool wrote:
> > > Hi Arnd,
> > >
> > > On Wed, Aug 03, 2016 at 08:52:48PM +0200, Arnd Bergmann wrote:
> > > > From my first look, it seems that all of lib/*.o is now getting linked
> > > > into vmlinux, while we traditionally leave out everything from lib/
> > > > that is not referenced.
> > > >
> > > > I also see a noticeable overhead in link time, the numbers are for
> > > > a cache-hot rebuild after a successful allyesconfig build, using a
> > > > 24-way Opteron@2.5Ghz, just relinking vmlinux:
> > > >
> > > > $ time make skj30 vmlinux # before
> > > > real 2m8.092s
> > > > user 3m41.008s
> > > > sys 0m48.172s
> > > >
> > > > $ time make skj30 vmlinux # after
> > > > real 4m10.189s
> > > > user 5m43.804s
> > > > sys 0m52.988s
> > >
> > > Is it better when using rcT instead of rcsT?
> >
> > It seems to be noticeably better for the clean rebuild case, though
> > not as good as the original:
> >
> > real 3m34.015s
> > user 5m7.104s
> > sys 0m49.172s
> >
> > I've also tried now with my own patch applied as well (linking
> > each drivers/*/built-in.o into vmlinux rather than having them
> > linked into drivers/built-in.o first), but that makes no
> > difference.
>
> I just want to come back to this, because I've subbmitted the thin
> archives kbuild patch, I wanted to make sure we're doing okay on
> ARM/ARM64. I cross compiled with my laptop.
>
> For ARM64 allyesconfig:
>
> After building then removing all built-in.o then rebuilding vmlinux:
> inclink
> time make ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- -j8 vmlinux
> real 1m18.977s
> user 2m14.512s
> sys 0m29.704s
>
> thinarc
> time make ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- -j8 vmlinux
> real 1m18.433s
> user 2m6.128s
> sys 0m28.372s
>
>
> Final ld time
> inclink
> real 0m4.005s
> user 0m3.464s
> sys 0m0.536s
>
> thinarc
> real 0m5.841s
> user 0m4.916s
> sys 0m0.916s
>
>
> Build directory size is of course much better (3953MB vs 5519MB).

Ok, looks great. Some downsides and some upsides here, but overall
I think this is a win.

>
> For ARM, defconfig
>
> After building then removing all built-in.o then rebuilding vmlinux:
> inclink
> real 0m19.593s
> user 0m22.372s
> sys 0m6.428s
>
> thinarc
> real 0m18.919s
> user 0m21.924s
> sys 0m6.400s
>
>
> Final ld time
> inclink
> real 0m0.378s
> user 0m0.304s
> sys 0m0.076s
>
> thinarc
> real 0m0.894s
> user 0m0.684s
> sys 0m0.200s

This also still seems fine.

> For both cases final link gets slower with thin archives. I guess there is some
> per-file overhead but I thought with --whole-archive it should not be that much
> slower. Still, overall time for main ar/ld phases comes out about the same in
> the end so I don't think it's too much problem. Unless ARM blows up significantly
> worse with a bigger config.

Unfortunately I think it does. I haven't tried your latest series yet,
but I think the total time for removing built-in.o and relinking went
up from around 4 minutes (already way too much) to 18 minutes for me.

> Linking with thin archives takes significantly more time in bfd hash lookup code.
> I haven't dug much further yet.

Can you try the ARM allyesconfig with thin archives? I'll follow up with two
patches: one to get ARM to link without thin archives, and one that I used
to get --gc-sections to work.

Arnd