linux-kernel - Re: [PATCH -v4 4/9] tracing: add static function tracer support for MIPS

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Date:	Thu, 22 Oct 2009 16:52:06 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Adam Nemet <anemet@...iumnetworks.com>
Cc:	David Daney <ddaney@...iumnetworks.com>, wuzhangjin@...il.com,
	Richard Sandiford <rdsandiford@...glemail.com>,
	linux-kernel@...r.kernel.org, linux-mips@...ux-mips.org,
	Thomas Gleixner <tglx@...utronix.de>,
	Ralf Baechle <ralf@...ux-mips.org>,
	Nicholas Mc Guire <der.herr@...r.at>
Subject: Re: [PATCH -v4 4/9] tracing: add static function tracer support
 for MIPS

On Thu, 2009-10-22 at 13:30 -0700, Adam Nemet wrote:
> Maybe scanning the instructions should stop at the beginning of the function
> based on the kernel's symbol table.  I am not sure if we can establish any
> other stopping condition without affecting performance too much.

How would we check the kernel symbol table? Remember, this happens at
_every_ function call.

> 
> Speaking of performance, -pg also affects the instruction scheduling freedom
> of the compiler in the prologue.  With profiling, we limit optimizations not
> to move instructions in and out of the prologue.
> 
> Also note that for functions invoked via tail call you won't get an exit
> event.  E.g. if bar is tail-called from foo:
> 
>   foo entered
>   bar entered
>   foo/bar exited
> 
> However, this is not MIPS-specific and you can always disable tail calls
> with -fno-optimize-sibling-calls.

The question is, would bar have a _mcount call? So far, we have not had
any issues with this on either x86 nor PPC.

/me knocks on wood.

-- Steve


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