linux-kernel - Re: [PATCH 0000/2297] [ANNOUNCE, RFC] "Fast Kernel Headers" Tree -v1: Eliminate the Linux kernel's "Dependency Hell"

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Date:   Sat, 8 Jan 2022 16:16:05 +0100
From:   Ingo Molnar <mingo@...nel.org>
To:     Nathan Chancellor <nathan@...nel.org>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "David S. Miller" <davem@...emloft.net>,
        Ard Biesheuvel <ardb@...nel.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        Al Viro <viro@...iv.linux.org.uk>, llvm@...ts.linux.dev
Subject: Re: [PATCH 0000/2297] [ANNOUNCE, RFC] "Fast Kernel Headers" Tree
 -v1: Eliminate the Linux kernel's "Dependency Hell"


* Nathan Chancellor <nathan@...nel.org> wrote:

> I tried to checkout at 9006a48618cc0cacd3f59ff053e6509a9af5cc18 to see if 
> I could reproduce that breakage there but the build errors out at that 
> change (I do see notes of bisection breakage in some of the commits) so I 
> assume that is expected.

Yeah, so the underlying problem is that these two commits want to be a 
single commit:

  # Commit #117
  headers/deps: Move task->thread_info to per_task()

  # Commit #106
  headers/deps: Move thread_info APIs to <linux/sched/thread_info_api.h>

As we can only switch ARM64's <asm/preempt.h> to use per_task() - which 
requires <linux/sched.h> - if we first fix & simplify <linux/sched.h>'s 
header dependencies, which is done to a sufficient level by:

  # Commit #556
  headers/deps: Optimize <linux/sched.h> dependencies, remove <linux/sched/thread_info_api_lowlevel.h> inclusion


So it's a catch-22, and quite a complication, and a bisection breakage 
distance of ~450 commits, with a lot of ordering assumptions & conflicts 
along the way, should we attempt to move the first two to later stages. :-/

But today I've restructured the tree, and the -v2-to-be tree is now fully 
bisectable on ARM64 too. :-)

There's a single, late per_cpu() conversion commit, after the first phase 
of <linux/sched.h> simplifications:

   headers/deps: Move task->thread_info to per_task()

I'd guess that either this one is that breaks SCS for you, or the ::thread 
conversion:

   headers/deps: per_task, arm64, x86: Convert task_struct::thread to a per_task() field

I've pushed out these fixes to the sched/headers branch a couple of minutes 
ago, and this will be part of the -v2 release as well.

Thanks,

	Ingo