Re: [PATCH][resubmit] x86: enable preemption in delay

Marin Mitov
2008-06-01T17:18:36Z

On Sunday 01 June 2008 07:25:17 pm Andi Kleen wrote:
> > static void delay_tsc(unsigned long loops)
> > {
> > - unsigned long bclock, now;
> > + unsigned prev, prev_1, now;
> > + unsigned left = loops;
> > + unsigned prev_cpu, cpu;
> > +
> > + preempt_disable();
> > + rdtscl(prev);
>
>
> The unsigneds should be probably u64 and the rdtsc rdtscll.
> Otherwise this will all overflow for longer waits on a very
> fast systems (e.g. a 5Ghz system wraps 32bit in ~1.1 seconds)
> Normally such delays shouldn't be that long, but why risk
> overflow even in extreme cases?

Yes in principles, but the overflow (that could happen between
rdtscl(prev) and rdtscl(now) is taken into account the same way
as in time_after()/time_before() macros, (differences only) see:

+ left -= now - prev;
.........
+ } while ((now-prev) < left);

If more than one overflow happen between rdtscl(prev) and
rdtscl(now) (the task is suspended for a long time between two
readings) all overflows after the first one will be lost. But the
patch was submitted to guaranty minimum udelay() initially.
Sure, I could change to u64 if we reach a concensus.

Best regards.

Marin Mitov

>
> -Andi
>