linux-kernel - Re: [PATCH] scheduler: fix x86 regression in native_sched

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Date:	Fri, 7 Dec 2007 13:11:07 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	stefano.brivio@...imi.it
Cc:	Nick Piggin <nickpiggin@...oo.com.au>, Robert Love <rml@...h9.net>,
	linux-kernel@...r.kernel.org, Dave Jones <davej@...hat.com>,
	"Rafael J. Wysocki" <rjw@...k.pl>, Michael Buesch <mb@...sch.de>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Len Brown <lenb@...nel.org>
Subject: Re: [PATCH] scheduler: fix x86 regression in native_sched_clock


* stefano.brivio@...imi.it <stefano.brivio@...imi.it> wrote:

>> It's a single CPU box, so sched_clock() jumping would still be 
>> problematic, no?
>
> I guess so. Definitely, it didn't look like a printk issue. Drivers 
> don't read logs, usually. But they got confused anyway (it seems that 
> udelay's get scaled or fail or somesuch - I can't test it right now, 
> will provide more feedback in a few hours).

no, i think it's just another aspect of the broken TSC on that hardware. 
Does the patch below improve things?

	Ingo

------------------->
Subject: x86: cpu_clock() based udelay
From: Ingo Molnar <mingo@...e.hu>

use cpu_clock() for TSC based udelay - it's more reliable than raw
TSC based delay loops.

Signed-off-by: Ingo Molnar <mingo@...e.hu>
---
 arch/x86/lib/delay_32.c |   20 ++++++++++++--------
 arch/x86/lib/delay_64.c |   27 ++++++++++++++++++---------
 2 files changed, 30 insertions(+), 17 deletions(-)

Index: linux/arch/x86/lib/delay_32.c
===================================================================
--- linux.orig/arch/x86/lib/delay_32.c
+++ linux/arch/x86/lib/delay_32.c
@@ -38,17 +38,21 @@ static void delay_loop(unsigned long loo
 		:"0" (loops));
 }
 
-/* TSC based delay: */
+/* cpu_clock() [TSC] based delay: */
 static void delay_tsc(unsigned long loops)
 {
-	unsigned long bclock, now;
+	unsigned long long start, stop, now;
+	int this_cpu;
+
+	preempt_disable();
+
+	this_cpu = smp_processor_id();
+	start = now = cpu_clock(this_cpu);
+	stop = start + loops;
+
+	while ((long long)(stop - now) > 0)
+		now = cpu_clock(this_cpu);
 
-	preempt_disable();		/* TSC's are per-cpu */
-	rdtscl(bclock);
-	do {
-		rep_nop();
-		rdtscl(now);
-	} while ((now-bclock) < loops);
 	preempt_enable();
 }
 
Index: linux/arch/x86/lib/delay_64.c
===================================================================
--- linux.orig/arch/x86/lib/delay_64.c
+++ linux/arch/x86/lib/delay_64.c
@@ -26,19 +26,28 @@ int read_current_timer(unsigned long *ti
 	return 0;
 }
 
-void __delay(unsigned long loops)
+/* cpu_clock() [TSC] based delay: */
+static void delay_tsc(unsigned long loops)
 {
-	unsigned bclock, now;
+	unsigned long long start, stop, now;
+	int this_cpu;
+
+	preempt_disable();
+
+	this_cpu = smp_processor_id();
+	start = now = cpu_clock(this_cpu);
+	stop = start + loops;
+
+	while ((long long)(stop - now) > 0)
+		now = cpu_clock(this_cpu);
 
-	preempt_disable();		/* TSC's are pre-cpu */
-	rdtscl(bclock);
-	do {
-		rep_nop(); 
-		rdtscl(now);
-	}
-	while ((now-bclock) < loops);
 	preempt_enable();
 }
+
+void __delay(unsigned long loops)
+{
+	delay_tsc(loops);
+}
 EXPORT_SYMBOL(__delay);
 
 inline void __const_udelay(unsigned long xloops)
--
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