Date: Tue, 2 Dec 2008 10:02:05 +0100
From: Ingo Molnar <mingo@elte.hu>
To: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Steven Rostedt <rostedt@goodmis.org>, Tim Bird <tim.bird@am.sony.com>,
       Linux Kernel <linux-kernel@vger.kernel.org>,
       Alexander van Heukelum <heukelum@mailshack.com>
Subject: Re: [PATCH] tracing/function-branch-tracer: support for x86-64
Message-ID: <20081202090205.GA11632@elte.hu>
References: <49347147.8070405@gmail.com>
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* Frederic Weisbecker <fweisbec@gmail.com> wrote:

> This patch implements the support for function branch tracer under x86-64.
> Both static and dynamic tracing are supported.

Fantastic stuff! :-)

> Small note: Ingo, I have only one test box and I had to install a 64 
> bits distro to make this patch. So I can't verify if it breaks 
> something in x86-32. I don't know what could be broken here but we 
> never know. For further patches, I will use a virtual machine to test 
> under 32.

that's OK. The patch looks fairly safe on the 32-bit side.

> This causes some small CPP conditional asm on arch/x86/kernel/ftrace.c 
> I wanted to use probe_kernel_read/write to make the return address 
> saving/patching code more generic but it causes tracing recursion.

it's this bit:

> +#ifdef CONFIG_X86_64
> +		"1: movq (%[parent_old]), %[old]\n"
> +		"2: movq %[return_hooker], (%[parent_replaced])\n"
> +#else
>  		"1: movl (%[parent_old]), %[old]\n"
>  		"2: movl %[return_hooker], (%[parent_replaced])\n"
> +#endif
>  		"   movl $0, %[faulted]\n"
>  
>  		".section .fixup, \"ax\"\n"
> @@ -476,8 +481,13 @@ void prepare_ftrace_return(unsigned long *parent, unsigned long self_addr)
>  		".previous\n"
>  
>  		".section __ex_table, \"a\"\n"
> +#ifdef CONFIG_X86_64
> +		"   .quad 1b, 3b\n"
> +		"   .quad 2b, 3b\n"
> +#else
>  		"   .long 1b, 3b\n"
>  		"   .long 2b, 3b\n"
> +#endif

i think we might want to introduce a few assembly helpers/defines to 
standardize such constructs - they are quite frequent. Something like:

		"   .ip_ptr 1b, 3b\n"
		"   .ip_ptr 2b, 3b\n"

(Cc:-ed Alexander and Cyrill who have done work in this area recently)

we might also introduce instruction helpers:

		"1: mov_ptr (%[parent_old]), %[old]\n"
		"2: mov_ptr %[return_hooker], (%[parent_replaced])\n"

and avoid the #ifdefs altogether.

> Note that arch/x86/process_64.c is not traced, as in X86-32. I first 
> thought __switch_to() was responsible of crashes during tracing because 
> I believed current task were changed inside but that's actually not the 
> case (actually yes, but not the "current" pointer).
> 
> So I will have to investigate to find the functions that harm here, to 
> enable tracing of the other functions inside (but there is no issue at 
> this time, while process_64.c stays out of -pg flags).

ok. You should take a look at arch/x86/include/asm/system.h's switch_to() 
macros - it has special stack switching smarts for context-switching.

the other special stack layout case is the starting of kernel threads - 
ret_from_fork and its details in process*.c.

> A little possible race condition is fixed inside this patch too. When 
> the tracer allocate a return stack dynamically, the current depth is 
> not initialized before but after. An interrupt could occur at this time 
> and, after seeing that the return stack is allocated, the tracer could 
> try to trace it with a random uninitialized depth. It's a prevention, 
> even if I hadn't problems with it.

> index 08b536a..1e9379d 100644
> --- a/kernel/trace/ftrace.c
> +++ b/kernel/trace/ftrace.c
> @@ -1673,8 +1673,8 @@ static int alloc_retstack_tasklist(struct ftrace_ret_stack **ret_stack_list)
>  		}
>  
>  		if (t->ret_stack == NULL) {
> -			t->ret_stack = ret_stack_list[start++];
>  			t->curr_ret_stack = -1;
> +			t->ret_stack = ret_stack_list[start++];
>  			atomic_set(&t->trace_overrun, 0);
>  		}
>  	} while_each_thread(g, t);

okay - the (optimization-)safe way to tell the compiler about such local 
CPU ordering information is:

diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
index 08b536a..f724996 100644
--- a/kernel/trace/ftrace.c
+++ b/kernel/trace/ftrace.c
@@ -1673,8 +1673,10 @@ static int alloc_retstack_tasklist(struct ftrace_ret_stack **ret_stack_list)
 		}
 
 		if (t->ret_stack == NULL) {
-			t->ret_stack = ret_stack_list[start++];
 			t->curr_ret_stack = -1;
+			/* Make sure IRQs see the -1 first: */
+			barrier();
+			t->ret_stack = ret_stack_list[start++];
 			atomic_set(&t->trace_overrun, 0);
 		}
 	} while_each_thread(g, t);

i changed the patch to do that.

All in one, great stuff!

	Ingo
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