Re: [PATCH] compiler.h, tracing: Remove CONFIG_PROFILE_ALL_BRANCHES

[Steven Rostedt]

On Fri, 19 Apr 2019 12:08:37 +0200
Ingo Molnar <mingo@xxxxxxxxxx> 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 Hereâ.