Re: [PATCH for 2.5] preemptible kernel

Keith Owens
2001-03-21T09:21:02Z

Nigel Gamble wrote:
> A task that has been preempted is on the run queue and can be
> rescheduled on a different CPU, so I can't see how a per-CPU counter
> would work. It seems to me that you would need a per run queue
> counter, like the example I gave in a previous posting.

Ouch. What about all the per cpu structures in the kernel, how do you
handle them if a preempted task can be rescheduled on another cpu?

int count[NR_CPUS], *p;
p = count+smp_processor_id(); /* start on cpu 0, &count[0] */
if (*p >= 1024) {
/* preempt here, reschedule on cpu 1 */
*p = 1; /* update cpu 0 count from cpu 1, oops */
}

Unless you find every use of a per cpu structure and wrap a spin lock
around it, migrating a task from one cpu to another is going to be a
source of wierd and wonderful errors. Since the main use of per cpu
structures is to avoid locks, adding a spin lock to every structure
will be a killer. Or have I missed something?