get_cycles() on i386

From: Joel Becker
Date: Tue Nov 04 2003 - 18:25:05 EST

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
E-mail: joel.becker@xxxxxxxxxx
Phone: (650) 506-8127
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at
Please read the FAQ at