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From: Joel Becker <>
	Marcelo Tosatti <>,
	john stultz <>
Subject: get_cycles() on i386
Date: Tue, 4 Nov 2003 15:22:53 -0800	[thread overview]
Message-ID: <> (raw)

	Certain distributions are building all of their SMP kernels
NUMA-aware.  This is great, as the kernels support boxes like the x440
with no trouble.  However, this implicitly disables CONFIG_X86_TSC.
While that is good for NUMA systems, and fine from a kernel timing
standpoint, it also eliminates any generic access to the TSC via
get_cycles().  With CONFIG_X86_TSC not defined, get_cycles() always
returns 0.
	Given that >95% of machines will not be x440s, this means that a
user of that kernel cannot access a high resolution timer via
get_cycles().  I don't want to have to litter my code with rdtscll()
when I managed to remove it!
	The proposed patch is trivial.  If the system has a TSC, it is
available get_cycles().  This makes no change to the other parts of the
kernel protected by CONFIG_X86_TSC.


diff -uNr ../kernel-2.4.21-4.0.1.EL/linux-2.4.21/include/asm-i386/timex.h linux-2.4.21/include/asm-i386/timex.h
--- ../kernel-2.4.21-4.0.1.EL/linux-2.4.21/include/asm-i386/timex.h	2002-11-28 15:53:15.000000000 -0800
+++ linux-2.4.21/include/asm-i386/timex.h	2003-11-04 11:33:08.000000000 -0800
@@ -40,7 +40,7 @@
 static inline cycles_t get_cycles (void)
-#ifndef CONFIG_X86_TSC
+#ifndef CONFIG_X86_HAS_TSC
 	return 0;
 	unsigned long long ret;


"Hey mister if you're gonna walk on water,
 Could you drop a line my way?"

Joel Becker
Senior Member of Technical Staff
Oracle Corporation
Phone: (650) 506-8127

             reply	other threads:[~2003-11-04 23:23 UTC|newest]

Thread overview: 7+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2003-11-04 23:22 Joel Becker [this message]
2003-11-04 23:27 ` get_cycles() on i386 john stultz
2003-11-04 23:54   ` Linus Torvalds
2003-11-05  2:53     ` Nick Piggin
2003-11-05  3:02       ` Nick Piggin
2003-11-05 13:35     ` Marcelo Tosatti
2003-12-05 15:55   ` Marcelo Tosatti

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