Re: get_cycles() on i386

[john stultz]

On Tue, 2003-11-04 at 15:22, Joel Becker wrote:
> Folks,
> 	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.

CONFIG_X86_TSC be the devil. Personally, I'd much prefer dropping the
compile time option and using dynamic detection. Something like (not
recently tested and i believe against 2.5.something, but you get the
idea):


diff -Nru a/include/asm-i386/timex.h b/include/asm-i386/timex.h
--- a/include/asm-i386/timex.h	Mon Feb 24 21:09:32 2003
+++ b/include/asm-i386/timex.h	Mon Feb 24 21:09:32 2003
@@ -40,14 +40,10 @@
 
 static inline cycles_t get_cycles (void)
 {
-#ifndef CONFIG_X86_TSC
-	return 0;
-#else
-	unsigned long long ret;
-
-	rdtscll(ret);
+	unsigned long long ret = 0;
+	if(cpu_has_tsc)
+		rdtscll(ret);
 	return ret;
-#endif
 }
 
 extern unsigned long cpu_khz;


thanks
-john


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