Message-ID: <8b5bc8ec-de33-45fe-964a-85a87947ccab@t-8ch.de>
Date: Fri, 2 Aug 2024 17:40:05 +0200
From: Thomas Weißschuh <linux@...ssschuh.net>
To: Jose Fernandez <jose.fernandez@...ux.dev>
Cc: Nathan Chancellor <nathan@...nel.org>, 
	Christian Heusel <christian@...sel.eu>, Masahiro Yamada <masahiroy@...nel.org>, 
	Nicolas Schier <nicolas@...sle.eu>, Peter Jung <ptr1337@...hyos.org>, linux-kbuild@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] kbuild: add debug package to pacman PKGBUILD

On 2024-08-01 22:27:45+0000, Jose Fernandez wrote:
> On 24/08/01 12:20PM, Nathan Chancellor wrote:
> > On Thu, Aug 01, 2024 at 08:53:54PM +0200, Thomas Weißschuh wrote:
> > > On 2024-08-01 11:36:37+0000, Nathan Chancellor wrote:
> > > > On Thu, Aug 01, 2024 at 07:29:40AM -0600, Jose Fernandez wrote:
> > > > > Add a new -debug package to the pacman PKGBUILD that will contain the
> > > > > vmlinux image for debugging purposes. This package depends on the
> > > > > -headers package and will be installed in /usr/src/debug/${pkgbase}.
> > > > > 
> > > > > The vmlinux image is needed to debug core dumps with tools like crash.
> > > > > 
> > > > > Signed-off-by: Jose Fernandez <jose.fernandez@...ux.dev>
> > > > > Reviewed-by: Peter Jung <ptr1337@...hyos.org>
> > > > 
> > > > This appears to add a non-trivial amount of time to the build when benchmarking
> > > > with Arch Linux's configuration (I measure 9% with hyperfine):
> > > 
> > > As nothing more is compiled, I guess this is just the additional
> > > packaging.
> > > 
> > > > Benchmark 1: pacman-pkg @ 21b136cc63d2 ("minmax: fix up min3() and max3() too")
> > > >   Time (mean ± σ):     579.541 s ±  0.585 s    [User: 22156.731 s, System: 3681.698 s]
> > > >   Range (min … max):   578.894 s … 580.033 s    3 runs
> > > > 
> > > > Benchmark 2: pacman-pkg @ c5af4db0563b ("kbuild: add debug package to pacman PKGBUILD")
> > > >   Time (mean ± σ):     633.419 s ±  0.972 s    [User: 22247.886 s, System: 3673.879 s]
> > > >   Range (min … max):   632.302 s … 634.070 s    3 runs
> > > > 
> > > > Summary
> > > >   pacman-pkg @ 21b136cc63d2 ("minmax: fix up min3() and max3() too") ran
> > > >     1.09 ± 0.00 times faster than pacman-pkg @ c5af4db0563b ("kbuild: add debug package to pacman PKGBUILD")
> 
> 
> Hi Nathan,
> Thanks for taking the time to benchmark the change. I did notice that the build 
> time for the -debug package felt a bit longer than I expected. I attributed it
> to having to package a large file.
> 
> > > > It would be nice to add some option to avoid building this package for
> > > > developers who may not want it (I know I personally would not want it
> > > > with that penalty because I do a lot of bisects) or maybe adding a
> > > > target to build this package with the rest like 'pacman-pkg-with-dbg' or
> > > > something? 
> 
> My original idea was to add a make config to enable the -debug package, ie:
> `make pacman-pkg DEBUG=1`. Your suggestion of a separate target is also a good
> idea. I don't have strong opinion on this, so I'm open to what you and Thomas
> prefer.

I'm also in favor of a variable.
The new target would be implemented through a variable anyways.
Instead of a specific variable it should be more generic.
The -api-headers package is most likely used even less than the debug one.

PACMAN_EXTRAPACKAGES="headers api-headers debug"
(Assuming the main kernel package will always be built)

> > > > Also, couldn't vmlinux be obtained from vmlinuz that already
> > > > exists in the main package via scripts/extract-vmlinux?
> 
> I didn't know you could do that. However, I suspect that more people will 
> encounter this issue and will want the vmlinux file available without knowing
> you can extract it from vmlinuz. I think it's better to have it available 
> explicitly in a package.

Agreed.

> > > 
> > > Jose:
> > > 
> > > In the vanilla PKGBUILD vmlinux is part of the linux-headers package:
> > > linux-headers /usr/lib/modules/6.10.2-arch1-1/build/vmlinux
> > > 
> > > Given that you already gate the new -debug package on CONFIG_MODULES,
> > > why not add the file to that package?
> 
> Hi Thomas,
> My only concern with that approach is that it won't address Nathan's concern
> about increasing the build time. I think an opt-in -debug package is the best
> solution to address this.
> 
> Additionally, Peter from CachyOS (cced) shared this merge request with me:
> https://gitlab.archlinux.org/archlinux/packaging/packages/linux/-/merge_requests/3
> 
> It looks like the vanilla PKGBUILD will start including a debug package as well.
> I think it may be good to align with the pattern they will be using.

This looks very good. But I it seems it will be some time pacman 7.0
will be available (especially in the derivate distros)
Also even with this the custom configuration mechanism would still be
used.

> > > 
> > > Then we could still discuss if it makes sense to gate vmlinux on
> > > CONFIG_MODULES or if -headers should be enabled unconditionally again.
> > 
> > I think having -headers be enabled/built by default is probably okay
> > since a regular user/builder might have external modules that need to be
> > rebuilt against the new kernel. However, I (and likely many other
> > upstream developers, however many use Arch) never build external modules
> > against my kernels, so it would be nice to have some way to opt out of
> > this penalty. I suspect it is just compressing down such a massive
> > vmlinux that causes this? I have not had access to a build log with my
> > testing, so unsure.
> > 
> > > Or we wait for somebody to show up and ask for it.
> > 
> > I won't insist though so if we want to wait for someone else, that's
> > fine with me too.
> 
> I will lean on your preference for how to proceed with this. I'm happy to make
> the changes you think are best. To me, it appears that the next iteration to 
> satisfy everyone is to make the -debug package opt-in via a new make target or
> a make config option.

Ack.

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