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Date:   Fri, 19 Apr 2019 09:04:40 -0400
From:   Steven Rostedt <rostedt@goodmis.org>
To:     Ingo Molnar <mingo@kernel.org>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Peter Anvin <hpa@zytor.com>,
        Julien Thierry <julien.thierry@arm.com>,
        Will Deacon <will.deacon@arm.com>,
        Andy Lutomirski <luto@amacapital.net>,
        Catalin Marinas <catalin.marinas@arm.com>,
        James Morse <james.morse@arm.com>, valentin.schneider@arm.com,
        Brian Gerst <brgerst@gmail.com>,
        Andrew Lutomirski <luto@kernel.org>,
        Borislav Petkov <bp@alien8.de>,
        Denys Vlasenko <dvlasenk@redhat.com>,
        Linux List Kernel Mailing <linux-kernel@vger.kernel.org>,
        Dmitry Vyukov <dvyukov@google.com>,
        Slavomir Kaslev <kaslevs@vmware.com>
Subject: Re: [PATCH] compiler.h, tracing: Remove CONFIG_PROFILE_ALL_BRANCHES
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On Fri, 19 Apr 2019 12:08:37 +0200
Ingo Molnar <mingo@kernel.org> wrote:

> > Can't you just have those same engineers look at perf data? This seems
> > like a very expensive and convoluted way of getting something.  

I haven't tried the perf data. How well does it work with running over
a 2 weeks to a month period? That's what I do yearly. Here's the
results of my last run:

  http://rostedt.homelinux.com/branches/gandalf-branches-2019/brach_all-2019-02-05

  http://rostedt.homelinux.com/branches/mammoth-branches-2019/branch_all-2019-01-02
  http://rostedt.homelinux.com/branches/mammoth-branches-2019/branch_all-2019-01-03
  http://rostedt.homelinux.com/branches/mammoth-branches-2019/branch_all-2019-01-17
  http://rostedt.homelinux.com/branches/mammoth-branches-2019/branch_all-2019-02-05

I have a cron job that runs nightly that copies the current state, and
if the machine reboots, it starts a new file (which is why there's
multiple files for mammoth - it rebooted).

> 
> So since no-one offered objections to using perf branch profiling instead 
> (which method allows so much more than CONFIG_PROFILE_ALL_BRANCHES: such 

I've never used it, so I have no idea if it is suitable or not.

> as profiling glibc and other user-space, or allowing to branch-profile 
> the kernel is an uninstrumented form not distorted by 
> CONFIG_PROFILE_ALL_BRANCHES code generation artifacts), lemme propose the 
> attached patch to remove if-tracing.
> 
> If the CONFIG_PROFILE_ALL_BRANCHES=y feature is required for anyone it 
> can still be reverted privately or maintained out of tree - no need to 
> burden the mainline kernel with this.

But is it a real burden? It's been in the kernel for over 10 years
with very little issue. Only when we do something drastic does it show
up, and it's usually a quick fix to get it working again.

I believe Josh even told me that it found a bug in the objtool code, so
it does still have benefit staying in the kernel even without people
using it for profiling.

Note, I'm in the middle of writing a LWN article about learning the
kernel from branch profiling and it would be a shame if it disappears
before I finish it.

-- Steve


> 
> I've build tested this and it Looks Perfect Here™.