linux-kernel - Re: [GIT PULL] perf fixes

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Message-ID: <4FE51325.8090408@linux.intel.com>
Date:	Fri, 22 Jun 2012 17:51:49 -0700
From:	Arjan van de Ven <arjan@...ux.intel.com>
To:	Steven Rostedt <rostedt@...dmis.org>
CC:	Hagen Paul Pfeifer <hagen@...u.net>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"H. Peter Anvin" <hpa@...ux.intel.com>
Subject: Re: [GIT PULL] perf fixes

On 6/22/2012 4:18 PM, Steven Rostedt wrote:
> How so? It's not a C function call (like the -finstrument-functions
> produces). It's an assembly function call. The only differences between
> having ftrace enabled and ftrace disabled with -mfentry is that you get
> a 5 byte nop at the start of each traceable function. Sure, it might put
> a little pressure on the icache, but from the benchmarks I've run, the
> impact has all been within the noise.
> 
> I've been told that it doesn't even hurt the pipeline. But I've Cc'd hpa
> and Arjan for their comments. How much impact does a 5 byte nop at the
> start of each function really have on the normal operations of the
> kernel?
> 

if it's truely an official nop, it will take decoder bandwidth obviously
(which can decode 3 to 4 instructions per cycle, depending on the
brand/model of CPU and the total size in bytes of these instructions).
likewise, at the end of the out of order pipeline, NOPs may take a
retirement slot (again 3 to 4 instructions per cycle)

icache is there as well, and if the NOP actually changes cpu flags (some
of the less fortunate ones do) that can create a false data dependency.


I would also worry about the compiler being able to inline a function
containing one of these, but that's a compiler thing, not a CPU type of
thing.

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