Date:   Thu, 6 Jan 2022 17:29:52 -0700
From:   Nathan Chancellor <nathan@kernel.org>
To:     Ingo Molnar <mingo@kernel.org>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org,
        Andrew Morton <akpm@linux-foundation.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        "David S. Miller" <davem@davemloft.net>,
        Ard Biesheuvel <ardb@kernel.org>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Jonathan Corbet <corbet@lwn.net>,
        Al Viro <viro@zeniv.linux.org.uk>, llvm@lists.linux.dev
Subject: Re: [PATCH 0000/2297] [ANNOUNCE, RFC] "Fast Kernel Headers" Tree
 -v1: Eliminate the Linux kernel's "Dependency Hell"
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On Tue, Jan 04, 2022 at 11:47:30AM +0100, Ingo Molnar wrote:
> > > With the fast-headers kernel that's down to ~36,000 lines of code, 
> > > almost a factor of 3 reduction:
> > > 
> > >   # fast-headers-v1:
> > >   kepler:~/mingo.tip.git> wc -l kernel/pid.i
> > >   35941 kernel/pid.i
> > 
> > Coming from someone who often has to reduce a preprocessed kernel source 
> > file with creduce/cvise to report compiler bugs, this will be a very 
> > welcomed change, as those tools will have to do less work, and I can get 
> > my reports done faster.
> 
> That's nice, didn't think of that side effect.
> 
> Could you perhaps measure this too, to see how much of a benefit it is?

As it turns out, I got an opportunity to measure this sooner rather than
later [1]. Using cvise [2] with an identical set of toolchains and
interestingness test [3], reducing net/core/skbuff.c took significantly
less time with the version from the fast-headers tree.

v5.16-rc8:

$ wc -l skbuff.i
105135 skbuff.i

$ time cvise test.fish skbuff.i
...
________________________________________________________
Executed in  114.02 mins    fish           external
   usr time  1180.43 mins   69.29 millis  1180.43 mins
   sys time  229.80 mins  248.11 millis  229.79 mins

fast-headers:

$ wc -l skbuff.i
78765 skbuff.i

$ time cvise test.fish skbuff.i
...
________________________________________________________
Executed in   47.38 mins    fish           external
   usr time  620.17 mins   32.78 millis  620.17 mins
   sys time  123.70 mins  122.38 millis  123.70 mins

I was not expecting that much of a difference but it somewhat makes
sense, as the tool spends less time eliminated unused code and the
compiler invocations will be incrementally quicker as the input becomes
smaller.

[1]: https://github.com/ClangBuiltLinux/linux/issues/1563
[2]: https://github.com/marxin/cvise
[3]: https://github.com/nathanchance/creduce-files/tree/61056fd763ae3bfb53ff0ae4c1d95550c7c0a5b7/cbl-1563

Cheers,
Nathan