According to this discussion:
http://lkml.org/lkml/2008/7/25/338
http://lkml.org/lkml/2008/7/26/72
Frame pointers do nothing useful on PowerPC, so lib/Kconfig.debug
makes CONFIG_FRAME_POINTER unselectable on PPC targets. But ftrace.h
requires CONFIG_FRAME_POINTER for CALLER_ADDR macros. Therefore
tracing is completely useless on PowerPC:
[...]
<idle>-0 0X.h3 2us+: 0:140:R + [000] 1733:120:S mvtsd
<idle>-0 0X.h3 9us+: 0 (0)
<idle>-0 0X..3 72us : 0 (0)
<idle>-0 0X..3 73us : 0:140:R ==> [000] 1733:120:R mvtsd
On PPC we can safely use __builtin_return_address(1..6) w/o frame
pointers, and with this patch the trace output looks OK:
[...]
<idle>-0 0X.h3 2us+: 0:140:R + [000] 1740:120:S mvtsd
<idle>-0 0X.h3 9us+: hrtimer_wakeup (__run_hrtimer)
<idle>-0 0X..3 87us : cpu_idle (__got2_end)
<idle>-0 0X..3 89us : 0:140:R ==> [000] 1740:120:R mvtsd
Signed-off-by: Anton Vorontsov <[email protected]>
---
Btw, on PowerPC tracing is also broken w/o "ring-buffer: fix alignment
problem" patch (currently collecting dust in the -tip tree, commit
082605de5f82eb692cc90f7fda071cc01bb5ac34). Any chance the fix go into
Linus' tree, to not waste other people's time bisecting and debugging
the problem? ;-)
For google: tracing, regression, "ring-buffer: move some metadata
into buffer page", commit abc9b56d66fbd4d93302ef4bf6fa726e1b8255f9,
answer is here.
include/linux/ftrace.h | 2 +-
1 files changed, 1 insertions(+), 1 deletions(-)
diff --git a/include/linux/ftrace.h b/include/linux/ftrace.h
index 9c5bc6b..13eba02 100644
--- a/include/linux/ftrace.h
+++ b/include/linux/ftrace.h
@@ -146,7 +146,7 @@ static inline void __ftrace_enabled_restore(int enabled)
#endif
}
-#ifdef CONFIG_FRAME_POINTER
+#if defined(CONFIG_FRAME_POINTER) || defined(CONFIG_PPC)
/* TODO: need to fix this for ARM */
# define CALLER_ADDR0 ((unsigned long)__builtin_return_address(0))
# define CALLER_ADDR1 ((unsigned long)__builtin_return_address(1))
--
1.5.6.5