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SubjectRe: [RFC PATCH 2/4] sched: Adding gang scheduling infrastrucure
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On Mon, 2011-12-19 at 14:04 +0530, Nikunj A. Dadhania wrote:

> +static void gang_sched_member(void *info)
> +{
> + struct task_group *tg = (struct task_group *) info;
> + struct cfs_rq *cfs_rq;
> + struct rq *rq;
> + int cpu;
> + unsigned long flags;
> +
> + cpu = smp_processor_id();
> + cfs_rq = tg->cfs_rq[cpu];
> + rq = cfs_rq->rq;
> +
> + raw_spin_lock_irqsave(&rq->lock, flags);
> +
> + /* Check if the runqueue has runnable tasks */
> + if (cfs_rq->nr_running) {
> + /* Favour this task group and set need_resched flag,
> + * added by following patches */

That's just plain insanity, patch 3 is all of 4 lines, why split that
and have an incomplete patch here?

> + }
> + raw_spin_unlock_irqrestore(&rq->lock, flags);
> +}
> +
> +#define GANG_SCHED_GRANULARITY 8

Why have this magical number to begin with?

> +void gang_sched(struct task_group *tg, struct rq *rq)
> +{
> + /* We do not gang sched here */
> + if (rq->gang_leader == 0 || !tg || tg->gang == 0)
> + return;
> +
> + /* Yes thats the leader */
> + if (rq->gang_leader == 1) {
> +
> + if (!in_interrupt() && !irqs_disabled()) {

How can this ever happen, schedule() can't be called from interrupt
context and post_schedule() ensures interrupts are enabled.

> + smp_call_function_many(rq->gang_cpumask,
> + gang_sched_member, tg, 0);

See this is just not going to happen..

> + rq->gang_schedule = 0;
> + }
> +
> + } else {
> + /*
> + * find the gang leader according to the span,
> + * currently we have it as 8cpu, this can be made
> + * dynamic
> + */
> + struct sched_domain *sd;
> + unsigned int count;
> + int i;
> +
> + for_each_domain(cpu_of(rq), sd) {
> + count = 0;
> + for_each_cpu(i, sched_domain_span(sd))
> + count++;

That's just incompetent; there's cpumask_weight(), also that's called
sd->span_weight.

> + if (count >= GANG_SCHED_GRANULARITY)
> + break;
> + }
> +
> + if (sd && cpu_of(rq) == domain_first_cpu(sd)) {
> + printk(KERN_INFO "Selected CPU %d as gang leader\n",
> + cpu_of(rq));
> + rq->gang_leader = 1;
> + rq->gang_cpumask = sched_domain_span(sd);
> + } else if (sd) {
> + /*
> + * A fellow cpu, it will receive gang
> + * initiations from the gang leader now
> + */
> + rq->gang_leader = 0;
> + }
> + }
> +}


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