linux-kernel - Re: [PATCH] perf bench: add --prefault option for causing page faults before benchmark

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Date:	Wed, 10 Nov 2010 10:29:45 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Hitoshi Mitake <mitake@....info.waseda.ac.jp>
Cc:	linux-kernel@...r.kernel.org, h.mitake@...il.com,
	Ma Ling <ling.ma@...el.com>, Zhao Yakui <yakui.zhao@...el.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Paul Mackerras <paulus@...ba.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH] perf bench: add --prefault option for causing page
 faults before benchmark


* Hitoshi Mitake <mitake@....info.waseda.ac.jp> wrote:

> This patch adds --prefault option to perf bench mem memcpy.
> If user specify this option to perf bench mem memcpy, overhead of
> page faults will be removed from the score of memcpy().
> 
> Example of usage:
> | % ./perf bench mem memcpy -l 500MB
> | # Running mem/memcpy benchmark...
> | # Copying 500MB Bytes from 0x7fc036749010 to 0x7fc055b4a010 ...
> |
> |      628.526821 MB/Sec
> | mitake@...1i:~/linux/.../tools/perf% ./perf bench mem memcpy -l 500MB --prefault
> | # Running mem/memcpy benchmark...
> | # Copying 500MB Bytes from 0x7ff1b45e2010 to 0x7ff1d39e3010 ...
> |
> |        4.849256 GB/Sec

Ok, looks rather useful.

We are rather close to being able to apply these bits. We need a resolution for the 
arch/x86/lib/memcpy_64.S details. The ugliest are these kinds of #ifdefs:

+#ifndef PERF_BENCH
 .Lmemcpy_e:
        .previous
+#endif

What happens if we keep that label in place?

This:

+#ifndef PERF_BENCH
 ENTRY(__memcpy)
 ENTRY(memcpy)
        CFI_STARTPROC
+#else
+	.globl  memcpy_x86_64_unrolled
+memcpy_x86_64_unrolled:
+#endif

Could be removed if you defined an ENTRY() macro in perf, right?

This:

+#ifndef PERF_BENCH
+
        CFI_ENDPROC
 ENDPROC(memcpy)
 ENDPROC(__memcpy)

Could be solved by defining ENDPROC()/etc. macros in perf, right?

We could remove this #ifdef:

+#ifndef PERF_BENCH
+
 #include <linux/linkage.h>

 #include <asm/cpufeature.h>
 #include <asm/dwarf2.h>

+#endif /* PERF_BENCH */

if you added empty linkage.h, cpufeature.h and dwarf2.h files as 
tools/perf/util/include/linux/linkage.h, tools/perf/util/include/asm/cpufeature.h.

That linkage.h file could even contain a short perf version of the ENTRY() macro, 
etc.

That way we can avoid having to touch arch/x86/lib/memcpy_64.S altogether.

Thanks,

	Ingo
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