[PATCH] compiler.h, tracing: Remove CONFIG_PROFILE_ALL_BRANCHES

[Ingo Molnar <mingo@kernel.org>]

* Peter Zijlstra <peterz@infradead.org> wrote:

> On Thu, Mar 07, 2019 at 01:18:41PM -0500, Steven Rostedt wrote:
> > On Thu, 7 Mar 2019 09:45:35 -0800
> > Linus Torvalds <torvalds@linux-foundation.org> wrote:
> > 
> > > On Thu, Mar 7, 2019 at 9:38 AM Peter Zijlstra <peterz@infradead.org> wrote:
> > > >
> > > > Also; it seems to me that something PT, or maybe even simply:
> > > >
> > > >   perf -e branches -e branch-misses
> > > >
> > > > would get you similar or sufficient information.  
> 
> > I currently have one of my engineers looking at the data and may be
> > sending patches soon. It's basically an entry level way to get into
> > kernel development. Note, no patch will be sent just because of the
> > data from the profiling. The task is to look at and understand the
> > code, and see if it can be optimized (with likely/unlikely or flow
> > changes). It's a way to get a better understanding of the kernel in
> > various locations. It is by no means "profiler said this, lets change
> > it." All changes must be rational, and make sense. The profiler is only
> > used to help find those places.
> 
> Can't you just have those same engineers look at perf data? This seems
> like a very expensive and convoluted way of getting something.

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

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

Thanks,

	Ingo

=============================>
From 3f689ed8a1555aabead90e015a47aefddd2a4e25 Mon Sep 17 00:00:00 2001
From: Ingo Molnar <mingo@kernel.org>
Date: Fri, 19 Apr 2019 11:42:43 +0200
Subject: [PATCH] compiler.h, tracing: Remove CONFIG_PROFILE_ALL_BRANCHES

Redefining 'if' in compiler.h was a hideously wonderful hack back in 2008 when
we merged it via:

  2bcd521a684c: ("trace: profile all if conditionals")

Meanwhile the 'wonderful' novelty part wore off a bit in that decade, and 
what remained is the 'hideous', mostly. ;-)

Meanwhile #2: we also merged perf and hardware branch tracing 
capabilities (branch-miss events, but also BTS and aux hw tracing),
which can collect similar data and so much more:

  $ perf -e branches -e branch-misses

So let's remove this constant source of headaches for good. Anyone truly 
interested in this feature can revert this commit and/or maintain it out 
of tree - but the upstream pain isn't really worth it.

Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
 include/asm-generic/vmlinux.lds.h           | 11 +----
 include/linux/compiler.h                    | 24 -----------
 kernel/trace/Kconfig                        | 17 --------
 kernel/trace/trace_branch.c                 | 66 -----------------------------
 tools/perf/tests/bpf-script-test-prologue.c |  9 ----
 5 files changed, 1 insertion(+), 126 deletions(-)

diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
index f8f6f04c4453..9c477b2136c2 100644
--- a/include/asm-generic/vmlinux.lds.h
+++ b/include/asm-generic/vmlinux.lds.h
@@ -126,14 +126,6 @@
 #define LIKELY_PROFILE()
 #endif
 
-#ifdef CONFIG_PROFILE_ALL_BRANCHES
-#define BRANCH_PROFILE()	__start_branch_profile = .;		\
-				KEEP(*(_ftrace_branch))			\
-				__stop_branch_profile = .;
-#else
-#define BRANCH_PROFILE()
-#endif
-
 #ifdef CONFIG_KPROBES
 #define KPROBE_BLACKLIST()	. = ALIGN(8);				      \
 				__start_kprobe_blacklist = .;		      \
@@ -266,8 +258,7 @@
 	__start___verbose = .;						\
 	KEEP(*(__verbose))                                              \
 	__stop___verbose = .;						\
-	LIKELY_PROFILE()		       				\
-	BRANCH_PROFILE()						\
+	LIKELY_PROFILE()						\
 	TRACE_PRINTKS()							\
 	BPF_RAW_TP()							\
 	TRACEPOINT_STR()
diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index d58aa0db05f9..c63105451c6a 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -48,30 +48,6 @@ void ftrace_likely_update(struct ftrace_likely_data *f, int val,
 #  define unlikely(x)	(__branch_check__(x, 0, __builtin_constant_p(x)))
 # endif
 
-#ifdef CONFIG_PROFILE_ALL_BRANCHES
-/*
- * "Define 'is'", Bill Clinton
- * "Define 'if'", Steven Rostedt
- */
-#define if(cond, ...) __trace_if( (cond , ## __VA_ARGS__) )
-#define __trace_if(cond) \
-	if (__builtin_constant_p(!!(cond)) ? !!(cond) :			\
-	({								\
-		int ______r;						\
-		static struct ftrace_branch_data			\
-			__aligned(4)					\
-			__section("_ftrace_branch")			\
-			______f = {					\
-				.func = __func__,			\
-				.file = __FILE__,			\
-				.line = __LINE__,			\
-			};						\
-		______r = !!(cond);					\
-		______r ? ______f.miss_hit[1]++ : ______f.miss_hit[0]++;\
-		______r;						\
-	}))
-#endif /* CONFIG_PROFILE_ALL_BRANCHES */
-
 #else
 # define likely(x)	__builtin_expect(!!(x), 1)
 # define unlikely(x)	__builtin_expect(!!(x), 0)
diff --git a/kernel/trace/Kconfig b/kernel/trace/Kconfig
index 8bd1d6d001d7..169c34e0f16d 100644
--- a/kernel/trace/Kconfig
+++ b/kernel/trace/Kconfig
@@ -366,23 +366,6 @@ config PROFILE_ANNOTATED_BRANCHES
 
 	  Note: this will add a significant overhead; only turn this
 	  on if you need to profile the system's use of these macros.
-
-config PROFILE_ALL_BRANCHES
-	bool "Profile all if conditionals" if !FORTIFY_SOURCE
-	select TRACE_BRANCH_PROFILING
-	imply CC_DISABLE_WARN_MAYBE_UNINITIALIZED  # avoid false positives
-	help
-	  This tracer profiles all branch conditions. Every if ()
-	  taken in the kernel is recorded whether it hit or miss.
-	  The results will be displayed in:
-
-	  /sys/kernel/debug/tracing/trace_stat/branch_all
-
-	  This option also enables the likely/unlikely profiler.
-
-	  This configuration, when enabled, will impose a great overhead
-	  on the system. This should only be enabled when the system
-	  is to be analyzed in much detail.
 endchoice
 
 config TRACING_BRANCHES
diff --git a/kernel/trace/trace_branch.c b/kernel/trace/trace_branch.c
index 3ea65cdff30d..be75301a9963 100644
--- a/kernel/trace/trace_branch.c
+++ b/kernel/trace/trace_branch.c
@@ -387,69 +387,3 @@ __init static int init_annotated_branch_stats(void)
 	return 0;
 }
 fs_initcall(init_annotated_branch_stats);
-
-#ifdef CONFIG_PROFILE_ALL_BRANCHES
-
-extern unsigned long __start_branch_profile[];
-extern unsigned long __stop_branch_profile[];
-
-static int all_branch_stat_headers(struct seq_file *m)
-{
-	seq_puts(m, "   miss      hit    % "
-		    "       Function                "
-		    "  File              Line\n"
-		    " ------- ---------  - "
-		    "       --------                "
-		    "  ----              ----\n");
-	return 0;
-}
-
-static void *all_branch_stat_start(struct tracer_stat *trace)
-{
-	return __start_branch_profile;
-}
-
-static void *
-all_branch_stat_next(void *v, int idx)
-{
-	struct ftrace_branch_data *p = v;
-
-	++p;
-
-	if ((void *)p >= (void *)__stop_branch_profile)
-		return NULL;
-
-	return p;
-}
-
-static int all_branch_stat_show(struct seq_file *m, void *v)
-{
-	struct ftrace_branch_data *p = v;
-	const char *f;
-
-	f = branch_stat_process_file(p);
-	return branch_stat_show_normal(m, p, f);
-}
-
-static struct tracer_stat all_branch_stats = {
-	.name = "branch_all",
-	.stat_start = all_branch_stat_start,
-	.stat_next = all_branch_stat_next,
-	.stat_headers = all_branch_stat_headers,
-	.stat_show = all_branch_stat_show
-};
-
-__init static int all_annotated_branch_stats(void)
-{
-	int ret;
-
-	ret = register_stat_tracer(&all_branch_stats);
-	if (!ret) {
-		printk(KERN_WARNING "Warning: could not register "
-				    "all branches stats\n");
-		return 1;
-	}
-	return 0;
-}
-fs_initcall(all_annotated_branch_stats);
-#endif /* CONFIG_PROFILE_ALL_BRANCHES */
diff --git a/tools/perf/tests/bpf-script-test-prologue.c b/tools/perf/tests/bpf-script-test-prologue.c
index 43f1e16486f4..1f048bd89b0d 100644
--- a/tools/perf/tests/bpf-script-test-prologue.c
+++ b/tools/perf/tests/bpf-script-test-prologue.c
@@ -10,15 +10,6 @@
 
 #include <uapi/linux/fs.h>
 
-/*
- * If CONFIG_PROFILE_ALL_BRANCHES is selected,
- * 'if' is redefined after include kernel header.
- * Recover 'if' for BPF object code.
- */
-#ifdef if
-# undef if
-#endif
-
 #define FMODE_READ		0x1
 #define FMODE_WRITE		0x2