4bsd fuzzy runq

[Julian Elischer]

Elijah Buck wrote:
> Hi,
> 
> I'm looking at the code for 4bsd fuzzy run queues in kern_switch.c
> 
> The relevant bit:
> 
> if (fuzz > 1) {
>     int count = fuzz;
>     int cpu = PCPU_GET(cpuid);
>     struct thread *td2;
>     td2 = td = TAILQ_FIRST(rqh);
>     while (count-- && td2) {
>         if (td->td_lastcpu == cpu) {

this should be td2. The bug is mine..
and I wondered why the code commited never had the same result as that 
I tested in another tree.

>             td = td2;
>             break;
>         }
>         td2 = TAILQ_NEXT(td2, td_runq);
>     }
> ...return(td)
> The purpose of this code appears to be to look through the runq to a depth
> defined by fuzz for a thread that was last run on the current cpu.
> 
> Here are the cases I see:
> 1.) if (td_lastcpu == cpu) on the first iteration then TAILQ_FIRST(rqh) is
> the selected thread (because of the break).
> 2.) if (td_lastcpu != cpu) on the first iteration then td is never set
> again, and so (td_lastcpu != cpu) is always true. So, again,
> TAILQ_FIRST(rqh) will be selected (because count will reach 0).
> 
> Which doesn't seem right (since that's what the else clause does). What am I
> missing here?
> 
> Thanks,
> Elijah
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