Re: [PATCH v6 4/7] arm64: Add ftrace support

On 03/14/2014 02:08 AM, Will Deacon wrote:

On Thu, Mar 13, 2014 at 10:13:47AM +0000, AKASHI Takahiro wrote:

...

This patch implements arm64 specific part to support function tracers, such as function (CONFIG_FUNCTION_TRACER), function_graph (CONFIG_FUNCTION_GRAPH_TRACER) and function profiler (CONFIG_FUNCTION_PROFILER).

With 'function' tracer, all the functions in the kernel are traced with timestamps in ${sysfs}/tracing/trace. If function_graph tracer is specified, call graph is generated.

The kernel must be compiled with -pg option so that _mcount() is inserted at the beginning of functions. This function is called on every function's entry as long as tracing is enabled. In addition, function_graph tracer also needs to be able to probe function's exit. ftrace_graph_caller() & return_to_handler do this by faking link register's value to intercept function's return path.

More details on architecture specific requirements are described in Documentation/trace/ftrace-design.txt.

[...]

You seem not to like this statement :-)

...

diff --git a/arch/arm64/kernel/entry-ftrace.S b/arch/arm64/kernel/entry-ftrace.S new file mode 100644 index 0000000..0ac31c8 --- /dev/null +++ b/arch/arm64/kernel/entry-ftrace.S @@ -0,0 +1,175 @@ +/*

• • arch/arm64/kernel/entry-ftrace.S
• • Copyright (C) 2013 Linaro Limited
• • Author: AKASHI Takahiro takahiro.akashi@linaro.org
• • This program is free software; you can redistribute it and/or modify
• • it under the terms of the GNU General Public License version 2 as
• • published by the Free Software Foundation.
• */

+#include <linux/linkage.h> +#include <asm/ftrace.h> +#include <asm/insn.h>

+/*

• • Gcc with -pg will put the following code in the beginning of each function:

• •  mov x0, x30
    

• •  bl _mcount
    

• • [function's body ...]
    

• • "bl _mcount" may be replaced to "bl ftrace_caller" or NOP if dynamic
• • ftrace is enabled.
• • Please note that x0 as an argument will not be used here because we can
• • get lr(x30) of instrumented function at any time by winding up call stack
• • as long as the kernel is compiled without -fomit-frame-pointer.
• • (or CONFIG_FRAME_POINTER, this is forced on arm64)
• • stack layout after mcount_enter in _mcount():
• • current sp/fp => 0:+-----+
• • in _mcount() | x29 | -> instrumented function's fp

• •                +-----+
    

• •                | x30 | -> _mcount()'s lr (= instrumented function's pc)
    

• • old sp => +16:+-----+
• • when instrumented | |
• • function calls | ... |
• • _mcount() | |

• •                |     |
    

• • instrumented => +xx:+--- --+
• • function's fp | x29 | -> parent's fp

• •                +-----+
    

• •                | x30 | -> instrumented function's lr (= parent's pc)
    

• •                +-----+
    

• •                | ... |
    

I guess it's just the diff that's misaligning your ASCII art here?

Yes, I think so. Misaligned due to "tab"

...

+/*

• • void return_to_handler(void)
• • Run ftrace_return_to_handler() before going back to parent.
• • @fp is checked against the value passed by ftrace_graph_caller()
• • only when CONFIG_FUNCTION_GRAPH_FP_TEST is enabled.a
• */

•   .global return_to_handler
  

+return_to_handler:

ENTRY(return_to_handler)

Fix it.

...

•   str     x0, [sp, #-16]!
  

•   mov     x0, x29                 //     parent's fp
  

•   bl      ftrace_return_to_handler// addr = ftrace_return_to_hander(fp);
  

•   mov     x30, x0                 // restore the original return address
  

•   ldr     x0, [sp], #16
  

•   ret
  

and an ENDPROC here.

Fix it. But please note that this (return_to_handler) is not a real function. Declaring it as ENDPROC is not very useful.

...

+#endif /* CONFIG_FUNCTION_GRAPH_TRACER */ diff --git a/arch/arm64/kernel/ftrace.c b/arch/arm64/kernel/ftrace.c new file mode 100644 index 0000000..a559ab8 --- /dev/null +++ b/arch/arm64/kernel/ftrace.c @@ -0,0 +1,64 @@ +/*

• • arch/arm64/kernel/ftrace.c
• • Copyright (C) 2013 Linaro Limited
• • Author: AKASHI Takahiro takahiro.akashi@linaro.org
• • This program is free software; you can redistribute it and/or modify
• • it under the terms of the GNU General Public License version 2 as
• • published by the Free Software Foundation.
• */

+#include <linux/ftrace.h> +#include <linux/swab.h> +#include <linux/uaccess.h>

+#include <asm/cacheflush.h> +#include <asm/ftrace.h> +#include <asm/insn.h>

+#ifdef CONFIG_FUNCTION_GRAPH_TRACER +/*

• • function_graph tracer expects ftrace_return_to_handler() to be called
• • on the way back to parent. For this purpose, this function is called
• • in _mcount() or ftrace_caller() to replace return address (*parent) on
• • the call stack to return_to_handler.
• • Note that @frame_pointer is used only for sanity check later.
• */

+void prepare_ftrace_return(unsigned long *parent, unsigned long self_addr,

•                      unsigned long frame_pointer)
  

+{

•   unsigned long return_hooker = (unsigned long)&return_to_handler;
  

•   unsigned long old;
  

•   struct ftrace_graph_ent trace;
  

•   int err;
  

•   if (unlikely(atomic_read(&current->tracing_graph_pause)))
  

•           return;
  

•   /*
  

•    * Note:
  

•    * No protection against faulting at *parent, which may be seen
  

•    * on other archs. It's unlikely on AArch64.
  

•    */
  

•   old = *parent;
  

•   *parent = return_hooker;
  

return_hook? People might take it personally otherwise ;)

I didn't know what "(return) hooker" means. Anyway, leave it as it is after you and Steven's discussion.

...

•   trace.func = self_addr;
  

in kernel/ftrace/ftrace.c there's an smb_wmb() between setting up tracing_graph_pause and setting the ret_stack with a comment saying:

 /* Make sure the tasks see the -1 first: */

Why don't we have a corresponding read-barrier here?

I think Steven answered it.

-Takahiro AKASHI

Will