linux-kernel - Re: [GIT PULL] RCU updates for v5.1

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Date:   Wed, 6 Mar 2019 08:08:36 +0100
From:   Ingo Molnar <mingo@...nel.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        "Paul E. McKenney" <paulmck@...ibm.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <a.p.zijlstra@...llo.nl>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [GIT PULL] RCU updates for v5.1


* Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> On Tue, Mar 5, 2019 at 4:03 AM Ingo Molnar <mingo@...nel.org> wrote:
> >
> > Please pull the latest core-rcu-for-linus git tree from:
> >
> >    git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git core-rcu-for-linus
> 
> Hmm. I think this made my build time explode.
> 
> I'm bisecting, but it looks like my allmodconfig builds went from ~31
> min to ~39 min with this pull.
> 
> NOTE! I'm  not saying that the kernel slowed down. Just the *build*
> (with the same kernel) slowed down.
> 
> It's odd, and maybe I did something wrong, so I'll have to
> double-check the numbers, but I thought I'd mention it in case
> somebody goes "Ahh, I did X to the include files, so now every
> compiler invocation probably sees all of xyz and that will slow things
> down enormously".

Could it be the locking tree?

In particular the atomics auto-generation slowed down the build before, 
but this build regression was thought to be fixed with:

  0cf264b3133d: locking/atomics: Check atomic headers with sha1sum

but maybe that's not the whole story?

Or it's an unreported slowdown related to the RCU tree, as you say.

Thanks,

	Ingo