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Subject[patch] x86: make delay_tsc() preemptible again

* Ingo Molnar <mingo@elte.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@elte.hu>

make delay_tsc() preemptible again.

Signed-off-by: Ingo Molnar <mingo@elte.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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