Hi!
> > + if (unlikely(cpu != smp_processor_id())) {
>
> Eeek, once you migrated you do this all the time. you need to update
> cpu here.
>
> > + if ((now-bclock) >= loops)
> > + break;
>
> Also this is really dangerous with unsynchronized TSCs. You might get
> migrated and return immediately because the TSC on the other CPU is
> far ahead.
>
> What you really want is something like the patch below, but we should
> reuse the sched_clock_cpu() thingy to make that simpler. Looking into
> that right now.
> @@ -40,17 +40,51 @@ static void delay_loop(unsigned long loops)
> :"0" (loops));
> }
>
> +/*
> + * 5 usec on a 1GHZ machine. Not necessarily correct, but not too long
> + * either.
> + */
> +#define TSC_MIGRATE_COUNT 5000
> +
> /* TSC based delay: */
> static void delay_tsc(unsigned long loops)
> {
> unsigned long bclock, now;
> + int cpu;
>
> - preempt_disable(); /* TSC's are per-cpu */
> + preempt_disable();
> + cpu = smp_processor_id();
> rdtscl(bclock);
> do {
> rep_nop();
> - rdtscl(now);
> - } while ((now-bclock) < loops);
> +
> + /* Allow RT tasks to run */
> + preempt_enable();
> + preempt_disable();
> +
> + /*
> + * It is possible that we moved to another CPU, and
> + * since TSC's are per-cpu we need to calculate
> + * that. The delay must guarantee that we wait "at
> + * least" the amount of time. Being moved to another
> + * CPU could make the wait longer but we just need to
> + * make sure we waited long enough. Rebalance the
> + * counter for this CPU.
> + */
> + if (unlikely(cpu != smp_processor_id())) {
> + if (loops <= TSC_MIGRATE_COUNT)
> + break;
> + cpu = smp_processor_id();
> + rdtscl(bclock);
> + loops -= TSC_MIGRATE_COUNT;
> + } else {
> + rdtscl(now);
> + if ((now - bclock) >= loops)
> + break;
> + loops -= (now - bclock);
> + bclock = now;
What happens with different cpus running on different frequencies...?
Cpufreq?
Pavel
--
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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