linux-kernel - Re: [PATCH v6 5/8] x86: do not trace __probe_kernel

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Date:	Tue, 14 Jan 2014 19:51:34 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Petr Mladek <pmladek@...e.cz>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Jiri Kosina <jkosina@...e.cz>, linux-kernel@...r.kernel.org,
	x86@...nel.org
Subject: Re: [PATCH v6 5/8] x86: do not trace __probe_kernel_read

On Tue, 10 Dec 2013 16:42:17 +0100
Petr Mladek <pmladek@...e.cz> wrote:

> probe_kernel_read is used when modifying function calls in ftrace_replace_code,
> see arch/x86/kernel/ftrace.c. On x86, the code is replaced using int3 guard.
> All functions are patched in parallel to reduce an expensive synchronization
> of all CPUs. The result is that all affected functions are called via
> ftrace_int3_handler during the process.
> 
> ftrace_int3_handler is relatively slow because it has to check whether
> it is responsible for the handled IP. Therefore we should not modify
> functions that used during patching. It would slowdown the patching.
> 
> I tried to switch between 7 tracers: blk, branch, function_graph, wakeup_rt,
> irqsoff, function, and nop. Every tracer has also been enabled and disabled.
> With 500 cycles, I got these times before this commit:
> 
>     real    18m2.477s     18m8.654s     18m9.196s
>     user    0m0.008s      0m0.008s      0m0.012s
>     sys     0m17.316s     0m17.104s     0m17.300s
> 
> and after this commit:
> 
>     real    16m14.390s    16m15.200s    16m19.632s
>     user    0m0.028s      0m0.024s	0m0.028s
>     sys     0m23.788s     0m23.812s	0m23.804s
> 

Actually no. The benefit of tracing a function outweighs the benefit
of speeding up the conversion. If you want a fast conversion, simply:

 echo __probe_kernel_read > /sys/kernel/debug/tracing/set_ftrace_notrace
 
Try your numbers again with the above line.

-- Steve

> Signed-off-by: Petr Mladek <pmladek@...e.cz>
> ---
>  mm/maccess.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/maccess.c b/mm/maccess.c
> index d53adf9ba84b..bed9ee854ea0 100644
> --- a/mm/maccess.c
> +++ b/mm/maccess.c
> @@ -18,7 +18,7 @@
>  long __weak probe_kernel_read(void *dst, const void *src, size_t size)
>      __attribute__((alias("__probe_kernel_read")));
>  
> -long __probe_kernel_read(void *dst, const void *src, size_t size)
> +long notrace __probe_kernel_read(void *dst, const void *src, size_t size)
>  {
>  	long ret;
>  	mm_segment_t old_fs = get_fs();

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