linux-kernel - Re: [RFC 3/3] tracing: set_ftrace

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Date:	Fri, 29 Apr 2011 11:16:33 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Jiri Olsa <jolsa@...hat.com>
Cc:	fweisbec@...il.com, linux-kernel@...r.kernel.org
Subject: Re: [RFC 3/3] tracing: set_ftrace_filter support

On Fri, 2011-04-29 at 16:43 +0200, Jiri Olsa wrote:
> diff --git a/include/linux/ftrace.h b/include/linux/ftrace.h
> index b12f5df..9ce3484 100644
> --- a/include/linux/ftrace.h
> +++ b/include/linux/ftrace.h
> @@ -164,6 +164,9 @@ struct dyn_ftrace {
>                 unsigned long           flags;
>                 struct dyn_ftrace       *newlist;
>         };
> +       unsigned long                   filter;
> +       unsigned long                   no_trace;
> +       unsigned long                   addr;
>         struct dyn_arch_ftrace          arch;
>  };
>   

Big Fricken NACK!

Absolutely not! The dyn_ftrace structure is allocated for almost every
single function in the kernel. Any function that can be traced has this
structure allocated.

On my box I have:

# cat /debug/tracing/dyn_ftrace_total_info 
32605 0 0

That's 32,605 functions that can be traced, which turns to be 32,605
dyn_ftrace structs allocated. Now this number is after all modules have
been loaded. You added 3 longs, on 64 bit boxes that's 24 bytes, you
just increased the function tracer footprint by 782,520 bytes!

I'm working around not touching that structure, and I will continue to
NAK any changes that increases its size. Actually, I did touch that
structure, but I did not increase the size of it.

-- Steve

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