linux-kernel - [patch] x86: make delay

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Date:	Fri, 16 Nov 2007 09:47:42 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Avi Kivity <avi@...ranet.com>
Cc:	Arjan van de Ven <arjan@...radead.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	akpm@...ux-foundation.org
Subject: [patch] x86: make delay_tsc() preemptible again


* Ingo Molnar <mingo@...e.hu> wrote:

> but that should not be needed in this case. Why doesnt the TSC using 
> delay loop simply poll the CPU it is on and fix up the TSC?

something like the patch below.

	Ingo

--------------->
Subject: x86: make delay_tsc() preemptible again
From: Ingo Molnar <mingo@...e.hu>

make delay_tsc() preemptible again.

Signed-off-by: Ingo Molnar <mingo@...e.hu>
---
 arch/x86/lib/delay_32.c |   28 +++++++++++++++++++++++-----
 arch/x86/lib/delay_64.c |   30 ++++++++++++++++++++++++------
 2 files changed, 47 insertions(+), 11 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 +38,35 @@ static void delay_loop(unsigned long loo
 		:"0" (loops));
 }
 
-/* TSC based delay: */
+/*
+ * TSC based delay:
+ *
+ * We are careful about preemption as TSC's are per-CPU.
+ */
 static void delay_tsc(unsigned long loops)
 {
-	unsigned long bclock, now;
+	unsigned long prev, now;
+	long left = loops;
+	int prev_cpu, cpu;
 
-	preempt_disable();		/* TSC's are per-cpu */
-	rdtscl(bclock);
+	preempt_disable();
+	rdtscl(prev);
 	do {
+		prev_cpu = smp_processor_id();
 		rep_nop();
+		preempt_enable();
+
+		preempt_disable();
+		cpu = smp_processor_id();
 		rdtscl(now);
-	} while ((now-bclock) < loops);
+		/*
+		 * If we preempted we skip this small amount of time:
+		 */
+		if (prev_cpu != cpu)
+			prev = now;
+		left -= now - prev;
+		prev = now;
+	} while (left > 0);
 	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
@@ -26,17 +26,35 @@ int read_current_timer(unsigned long *ti
 	return 0;
 }
 
+/*
+ * TSC based delay:
+ *
+ * We are careful about preemption as TSC's are per-CPU.
+ */
 void __delay(unsigned long loops)
 {
-	unsigned bclock, now;
+	unsigned long prev, now;
+	long left = loops;
+	int prev_cpu, cpu;
 
-	preempt_disable();		/* TSC's are pre-cpu */
-	rdtscl(bclock);
+	preempt_disable();
+	rdtscl(prev);
 	do {
-		rep_nop(); 
+		prev_cpu = smp_processor_id();
+		rep_nop();
+		preempt_enable();
+
+		preempt_disable();
+		cpu = smp_processor_id();
 		rdtscl(now);
-	}
-	while ((now-bclock) < loops);
+		/*
+		 * If we preempted we skip this small amount of time:
+		 */
+		if (prev_cpu != cpu)
+			prev = now;
+		left -= now - prev;
+		prev = now;
+	} while (left > 0);
 	preempt_enable();
 }
 EXPORT_SYMBOL(__delay);
-
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