linux-kernel - Re: [GIT PULL] x86/cpu changes for v2.6.34

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Message-ID: <1267482223.10871.19.camel@gandalf.stny.rr.com>
Date:	Mon, 01 Mar 2010 17:23:43 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-kernel@...r.kernel.org, "H. Peter Anvin" <hpa@...or.com>,
	Borislav Petkov <borislav.petkov@....com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [GIT PULL] x86/cpu changes for v2.6.34

On Mon, 2010-03-01 at 14:42 -0500, Steven Rostedt wrote:

> As Frederic has said you can use 'ftrace=function_graph' on the kernel
> command line. It will be initialized in early_initcall (which I believe
> is before CPUs are set up. Then add a tracing_off() after the trouble
> code. You can make the trace buffers bigger with the kernel command
> line:
> 
> 	trace_buf_size=10000000
> 
> The above will make the trace buffer 10Meg per CPU. Unlike the
> "buffer_size_kb" file, this number is in bytes, even though it will
> round to the nearest page. (I probably should make this into kb, and
> rename it to trace_buf_size_kb, and deprecate trace_buf_size).
> 
> Then you can cat out /debug/tracing/trace, and search for large
> latencies in the timestamps.

I just tried the above and it doesn't work. The ring buffer gets
allocated with the early_initcall(), so trace_printk()'s will work. But
the function and function graph tracers don't get registered until the
device_initcall().

If you are still interested, this patch will allow you to run the
function graph tracer before smp_init(). You still need to add
"ftrace=function_graph" on the kernel command line.

It's a hack, but I tried it out and it worked.

-- Steve



diff --git a/init/main.c b/init/main.c
index 4cb47a1..b334663 100644
--- a/init/main.c
+++ b/init/main.c
@@ -868,8 +868,15 @@ static int __init kernel_init(void * unused)
 	do_pre_smp_initcalls();
 	start_boot_trace();
 
+	{
+		int init_graph_trace(void);
+		init_graph_trace();
+	}
+	trace_printk("start\n");
 	smp_init();
 	sched_init_smp();
+	trace_printk("end\n");
+	tracing_off();
 
 	do_basic_setup();
 
diff --git a/kernel/trace/trace_functions_graph.c b/kernel/trace/trace_functions_graph.c
index aaf580c..f18cad8 100644
--- a/kernel/trace/trace_functions_graph.c
+++ b/kernel/trace/trace_functions_graph.c
@@ -1214,11 +1214,11 @@ static struct tracer graph_trace __read_mostly = {
 #endif
 };
 
-static __init int init_graph_trace(void)
+__init int init_graph_trace(void)
 {
 	max_bytes_for_cpu = snprintf(NULL, 0, "%d", nr_cpu_ids - 1);
 
 	return register_tracer(&graph_trace);
 }
 
-device_initcall(init_graph_trace);
+//device_initcall(init_graph_trace);


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