linux-kernel - Re: [PATCH] kprobes - do not allow optimized kprobes in entry code

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Message-ID: <20110221115452.GA1919@jolsa.brq.redhat.com>
Date:	Mon, 21 Feb 2011 12:54:52 +0100
From:	Jiri Olsa <jolsa@...hat.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Frédéric Weisbecker <fweisbec@...il.com>,
	"H. Peter Anvin" <hpa@...or.com>, ananth@...ibm.com,
	davem@...emloft.net, linux-kernel@...r.kernel.org,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Eric Dumazet <eric.dumazet@...il.com>,
	"2nddept-manager@....hitachi.co.jp" 
	<2nddept-manager@....hitachi.co.jp>
Subject: Re: [PATCH] kprobes - do not allow optimized kprobes in entry code

On Sun, Feb 20, 2011 at 01:59:48PM +0100, Ingo Molnar wrote:
> 
> * Masami Hiramatsu <masami.hiramatsu.pt@...achi.com> wrote:
> 
> > Thanks, I've also tested. (But my machine has no L1-icache-prefetches* support) 
> > What I can tell is both of L1-icache-load and L1-icache-load-misses is reduced by 
> > the patch. ;-)
> 
> That's actually a pretty interesting result: it means that compressing entry code 
> into a single section compresses the icache footprint in a measurable way. The 
> icache miss rate went down by about 6%:
> 
> >          1,234,272 L1-icache-load-misses      ( +-   0.105% )
> >          1,155,816 L1-icache-load-misses      ( +-   0.113% )
> 
> Which, assuming that there's no kernel build and bootup related skew effect that is 
> larger than 2-3% means that this is an improvement.
> 
> perf feature request: would be nice if it was able to do:
> 
> 	perf stat --record ...
> 	perf diff
> 
> and it would show a comparison of the two runs.
> 
> In hindsight it makes sense: the patch probably reduced the fragmentation of the 
> icache for this workload.
> 
> But it's still surprising :-)
> 
> Mind splitting the patch up into two parts, the first one that does the performance 
> optimization intentionally (with numbers, explanation, etc.), the second one that 
> uses the new section for kprobes exclusion?
np, I'll send v2 shortly

thanks,
jirka

> 
> Thanks,
> 
> 	Ingo
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