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SubjectRe: [patch] x86: make delay_tsc() preemptible again
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On Fri, 2007-11-16 at 09:47 +0100, Ingo Molnar wrote:
> * 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();

Why not have the rep_nop() here between the enable, and disable ?

> +
> + preempt_disable();
> + cpu = smp_processor_id();
> rdtscl(now);
> - } while ((now-bclock) < loops);
> + /*
> + * If we preempted we skip this small amount of time:
^ migrated, perhaps?

> + */
> + if (prev_cpu != cpu)
> + prev = now;
> + left -= now - prev;
> + prev = now;
> + } while (left > 0);
> preempt_enable();
> }


Otherwise, looks like a very nice patch :-)


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