linux-kernel - Re: [PATCH v7 04/17] tracing: add static function tracer support for MIPS

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Date:	Wed, 11 Nov 2009 04:13:25 +0100
From:	Ralf Baechle <ralf@...ux-mips.org>
To:	Wu Zhangjin <wuzhangjin@...il.com>
Cc:	David Daney <ddaney@...iumnetworks.com>, linux-mips@...ux-mips.org,
	linux-kernel@...r.kernel.org, zhangfx@...ote.com, zhouqg@...il.com,
	rostedt@...dmis.org, Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...e.hu>,
	Nicholas Mc Guire <der.herr@...r.at>,
	Richard Sandiford <rdsandiford@...glemail.com>,
	Patrik Kluba <kpajko79@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Michal Simek <monstr@...str.eu>
Subject: Re: [PATCH v7 04/17] tracing: add static function tracer support
	for MIPS

On Wed, Nov 11, 2009 at 10:42:31AM +0800, Wu Zhangjin wrote:

> > -mlong-calls really degrades performance.  I have seen things like 6% 
> > drop in network packet forwarding rates with -mlong-calls.
> > 
> 
> so much drop? seems only two instructions added for it: lui, addi. from
> this view point, I think the -fno-omit-frame-pointer(add, sd, move...)
> will also bring with much drop.

The calling sequence is quite badly bloated.  Example:

Normal 32/64-bit subroutine call:

	jal	symbol

32-bit with -mlong-call:

	lui	$25, %hi(foo)
	addiu	$25, %lo(foo)
	jalr	$25

64-bit with -mlong-call:

	lui	$25, %highest(foo)
	lui	$2, %hi(foo)
	daddiu	$25, %higher(foo)
	daddiu	$2, %lo(foo)
	dsll	$25, 32
	daddu	$25, $2
	jalr	$25

So not considering the possible cost of the delay slot that's 1 vs. 3 vs. 7
instructions.  Last I checked ages ago gcc didn't apropriately consider this
cost when generating -mlong-calls code and Linux these days also is
optimized under the assumption that subroutine calls are cheap.

It's time that we get a -G optimization that works for the kernel; it would
allow to cut down the -mlong-calls calling sequence to just:

	lw/ld	$25, offset($gp)
	jalr	$25

> It's time to remove them? -mlong-calls, -fno-omit-frame-pointer.
> 
> > It would be better to fix all the tools so that they could handle both 
> > -mlong-calls and -mno-long-calls code.
> > 
> 
> It's totally possible, will try to make it work later. I just wanted the
> stuff simple, but if it really brings us with much drop, it's time to
> fix it.

  Ralf
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