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From: john stultz <johnstul@us.ibm.com>
To: Joel Becker <Joel.Becker@oracle.com>
Cc: lkml <linux-kernel@vger.kernel.org>,
	Marcelo Tosatti <marcelo.tosatti@cyclades.com>
Subject: Re: get_cycles() on i386
Date: 04 Nov 2003 15:27:43 -0800	[thread overview]
Message-ID: <1067988463.11437.115.camel@cog.beaverton.ibm.com> (raw)
In-Reply-To: <20031104232252.GG5792@ca-server1.us.oracle.com>

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



  reply	other threads:[~2003-11-04 23:29 UTC|newest]

Thread overview: 7+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2003-11-04 23:22 Joel Becker
2003-11-04 23:27 ` john stultz [this message]
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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