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Subject[patch 4/5] sched: RCU sched domains
4/5
One of the problems with the multilevel balance-on-fork/exec is that it
needs to jump through hoops to satisfy sched-domain's locking semantics
(that is, you may traverse your own domain when not preemptable, and
you may traverse others' domains when holding their runqueue lock).

balance-on-exec had to potentially migrate between more than one CPU before
finding a final CPU to migrate to, and balance-on-fork needed to potentially
take multiple runqueue locks.

So bite the bullet and make sched-domains go completely RCU. This actually
simplifies the code quite a bit.

Signed-off-by: Nick Piggin <nickpiggin@yahoo.com.au>

Index: linux-2.6/kernel/sched.c
===================================================================
--- linux-2.6.orig/kernel/sched.c 2005-04-05 16:39:14.000000000 +1000
+++ linux-2.6/kernel/sched.c 2005-04-05 18:39:05.000000000 +1000
@@ -825,22 +825,12 @@ inline int task_curr(const task_t *p)
}

#ifdef CONFIG_SMP
-enum request_type {
- REQ_MOVE_TASK,
- REQ_SET_DOMAIN,
-};
-
typedef struct {
struct list_head list;
- enum request_type type;

- /* For REQ_MOVE_TASK */
task_t *task;
int dest_cpu;

- /* For REQ_SET_DOMAIN */
- struct sched_domain *sd;
-
struct completion done;
} migration_req_t;

@@ -862,7 +852,6 @@ static int migrate_task(task_t *p, int d
}

init_completion(&req->done);
- req->type = REQ_MOVE_TASK;
req->task = p;
req->dest_cpu = dest_cpu;
list_add(&req->list, &rq->migration_queue);
@@ -4365,17 +4354,9 @@ static int migration_thread(void * data)
req = list_entry(head->next, migration_req_t, list);
list_del_init(head->next);

- if (req->type == REQ_MOVE_TASK) {
- spin_unlock(&rq->lock);
- __migrate_task(req->task, cpu, req->dest_cpu);
- local_irq_enable();
- } else if (req->type == REQ_SET_DOMAIN) {
- rq->sd = req->sd;
- spin_unlock_irq(&rq->lock);
- } else {
- spin_unlock_irq(&rq->lock);
- WARN_ON(1);
- }
+ spin_unlock(&rq->lock);
+ __migrate_task(req->task, cpu, req->dest_cpu);
+ local_irq_enable();

complete(&req->done);
}
@@ -4606,7 +4587,6 @@ static int migration_call(struct notifie
migration_req_t *req;
req = list_entry(rq->migration_queue.next,
migration_req_t, list);
- BUG_ON(req->type != REQ_MOVE_TASK);
list_del_init(&req->list);
complete(&req->done);
}
@@ -4903,10 +4883,7 @@ static int __devinit sd_parent_degenerat
*/
void __devinit cpu_attach_domain(struct sched_domain *sd, int cpu)
{
- migration_req_t req;
- unsigned long flags;
runqueue_t *rq = cpu_rq(cpu);
- int local = 1;
struct sched_domain *tmp;

/* Remove the sched domains which do not contribute to scheduling. */
@@ -4923,24 +4900,7 @@ void __devinit cpu_attach_domain(struct

sched_domain_debug(sd, cpu);

- spin_lock_irqsave(&rq->lock, flags);
-
- if (cpu == smp_processor_id() || !cpu_online(cpu)) {
- rq->sd = sd;
- } else {
- init_completion(&req.done);
- req.type = REQ_SET_DOMAIN;
- req.sd = sd;
- list_add(&req.list, &rq->migration_queue);
- local = 0;
- }
-
- spin_unlock_irqrestore(&rq->lock, flags);
-
- if (!local) {
- wake_up_process(rq->migration_thread);
- wait_for_completion(&req.done);
- }
+ rq->sd = sd;
}

/* cpus with isolated domains */
@@ -5215,6 +5175,7 @@ static int update_sched_domains(struct n
case CPU_DOWN_PREPARE:
for_each_online_cpu(i)
cpu_attach_domain(NULL, i);
+ synchronize_kernel();
arch_destroy_sched_domains();
return NOTIFY_OK;
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