[PATCH] bug: Introduce CONFIG_DEBUG_BUGVERBOSE_EXTRA=y to also log warning conditions

[Ingo Molnar]

* Ingo Molnar <mingo@xxxxxxxxxx> wrote:

> > >    #define SCHED_WARN_ON(x)      WARN_ONCE(x, #x)
> > > 
> > > Because SCHED_WARN_ON() would output the 'x' condition
> > > as well, while WARN_ONCE() will only show a backtrace.
> > > 
> > > Hopefully these are rare enough to not really matter.
> > > 
> > > If it does, we should probably introduce a new WARN_ON()
> > > variant that outputs the condition in stringified form,
> > > or improve WARN_ON() itself.
> > 
> > So those strings really were useful, trouble is WARN_ONCE() generates
> > utter crap code compared to WARN_ON_ONCE(), but since SCHED_DEBUG that
> > doesn't really matter.
> 
> Why wouldn't it matter? CONFIG_SCHED_DEBUG was turned on for 99.9999% 
> of Linux users, ie. we generated crap code for most of our users.
> 
> And as a side effect of using the standard WARN_ON_ONCE() primitive we 
> now generate better code, at the expense of harder to interpret debug 
> output, right?
> 
> Ie. CONFIG_SCHED_DEBUG has obfuscated crappy code generation under the 
> "it's only debugging code" pretense, right?

So, to argue this via code, we'd like to have something like the patch below?

When enabled it will warn in the following fashion:

  static void super_perfect_kernel_function(void *ptr)
  {
	...
	WARN_ON_ONCE(ptr == 0 && 1);
	...
  }


  ------------[ cut here ]------------
  FAIL: 'ptr == 0 && 1' is true
  WARNING: CPU: 0 PID: 0 at kernel/sched/core.c:8511 sched_init+0x44/0x430
  ...

But the real question is, how do we keep distros from enabling 
CONFIG_DEBUG_BUGVERBOSE_EXTRA=y?

It does bloat the defconfig by about +144k .text and ~64k data, so 
maybe that's deterrence enough.

The BSS shift is due to it not using the clever x86 U2D tricks, right?

Thanks,

	Ingo

=================>
From: Ingo Molnar <mingo@xxxxxxxxxx>
Date: Tue, 25 Mar 2025 11:35:20 +0100
Subject: [PATCH] bug: Introduce CONFIG_DEBUG_BUGVERBOSE_EXTRA=y to also log warning conditions

      text         data	    bss	     dec	    hex	filename
  29522704	7926322	1389904	38838930	250a292	vmlinux.before
  29667392	8017958	1363024	39048374	253d4b6	vmlinux.after

Totally-Not-Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
---
 include/asm-generic/bug.h |  7 +++++++
 kernel/sched/core.c       |  2 ++
 lib/Kconfig.debug         | 12 ++++++++++++
 3 files changed, 21 insertions(+)

diff --git a/include/asm-generic/bug.h b/include/asm-generic/bug.h
index 387720933973..5475258a99dc 100644
--- a/include/asm-generic/bug.h
+++ b/include/asm-generic/bug.h
@@ -92,6 +92,11 @@ void warn_slowpath_fmt(const char *file, const int line, unsigned taint,
 		       const char *fmt, ...);
 extern __printf(1, 2) void __warn_printk(const char *fmt, ...);
 
+#ifdef CONFIG_DEBUG_BUGVERBOSE_EXTRA
+#define WARN_ON_ONCE(condition)						\
+	DO_ONCE_LITE_IF(condition, WARN, 1, "FAIL: '%s' is true", #condition)
+#endif
+
 #ifndef __WARN_FLAGS
 #define __WARN()		__WARN_printf(TAINT_WARN, NULL)
 #define __WARN_printf(taint, arg...) do {				\
@@ -107,6 +112,7 @@ extern __printf(1, 2) void __warn_printk(const char *fmt, ...);
 		__WARN_FLAGS(BUGFLAG_NO_CUT_HERE | BUGFLAG_TAINT(taint));\
 		instrumentation_end();					\
 	} while (0)
+#ifndef WARN_ON_ONCE
 #define WARN_ON_ONCE(condition) ({				\
 	int __ret_warn_on = !!(condition);			\
 	if (unlikely(__ret_warn_on))				\
@@ -115,6 +121,7 @@ extern __printf(1, 2) void __warn_printk(const char *fmt, ...);
 	unlikely(__ret_warn_on);				\
 })
 #endif
+#endif
 
 /* used internally by panic.c */
 
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 87540217fc09..71bf94bf68f8 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -8508,6 +8508,8 @@ void __init sched_init(void)
 	unsigned long ptr = 0;
 	int i;
 
+	WARN_ON_ONCE(ptr == 0 && 1);
+
 	/* Make sure the linker didn't screw up */
 #ifdef CONFIG_SMP
 	BUG_ON(!sched_class_above(&stop_sched_class, &dl_sched_class));
diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index b1b92a9a8f24..88f215f712f8 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -206,6 +206,18 @@ config DEBUG_BUGVERBOSE
 	  of the BUG call as well as the EIP and oops trace.  This aids
 	  debugging but costs about 70-100K of memory.
 
+config DEBUG_BUGVERBOSE_EXTRA
+	bool "Extra verbose WARN_ON() reporting" if DEBUG_BUGVERBOSE
+	default n
+	help
+	  Say Y here to make WARN_ON() warnings extra verbose, printing
+	  the condition they warn about.
+
+	  This aids debugging but uses up some memory and causes some
+	  runtime overhead due to worse code generation.
+
+	  If unsure, say N.
+
 endmenu # "printk and dmesg options"
 
 config DEBUG_KERNEL