[PATCH] sched: RCU domains

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.

From: Ingo Molnar <mingo@elte.hu>

schedstats RCU fix, and a nice comment on for_each_domain, from Ingo.

Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Nick Piggin <nickpiggin@yahoo.com.au>
Acked-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 15 insertions, 45 deletions

diff --git a/kernel/sched.c b/kernel/sched.c
index ef32389ee768..54ce787b6207 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -260,8 +260,15 @@ struct runqueue {
 
 static DEFINE_PER_CPU(struct runqueue, runqueues);
 
+/*
+ * The domain tree (rq->sd) is protected by RCU's quiescent state transition.
+ * See update_sched_domains: synchronize_kernel for details.
+ *
+ * The domain tree of any CPU may only be accessed from within
+ * preempt-disabled sections.
+ */
 #define for_each_domain(cpu, domain) \
-        for (domain = cpu_rq(cpu)->sd; domain; domain = domain->parent)
+for (domain = rcu_dereference(cpu_rq(cpu)->sd); domain; domain = domain->parent)
 
 #define cpu_rq(cpu)             (&per_cpu(runqueues, (cpu)))
 #define this_rq()               (&__get_cpu_var(runqueues))
@@ -395,6 +402,7 @@ static int show_schedstat(struct seq_file *seq, void *v)
 
 #ifdef CONFIG_SMP
                 /* domain-specific stats */
+                preempt_disable();
                 for_each_domain(cpu, sd) {
                         enum idle_type itype;
                         char mask_str[NR_CPUS];
@@ -419,6 +427,7 @@ static int show_schedstat(struct seq_file *seq, void *v)
                             sd->sbf_cnt, sd->sbf_balanced, sd->sbf_pushed,
                             sd->ttwu_wake_remote, sd->ttwu_move_affine, sd->ttwu_move_balance);
                 }
+                preempt_enable();
 #endif
         }
         return 0;
@@ -824,22 +833,12 @@ 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;
 
@@ -861,7 +860,6 @@ static int migrate_task(task_t *p, int dest_cpu, migration_req_t *req)
         }
 
         init_completion(&req->done);
-        req->type = REQ_MOVE_TASK;
         req->task = p;
         req->dest_cpu = dest_cpu;
         list_add(&req->list, &rq->migration_queue);
@@ -4378,17 +4376,9 @@ 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);
         }
@@ -4619,7 +4609,6 @@ static int migration_call(struct notifier_block *nfb, unsigned long action,
                         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);
                 }
@@ -4800,10 +4789,7 @@ static int __devinit sd_parent_degenerate(struct sched_domain *sd,
  */
 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. */
@@ -4820,24 +4806,7 @@ void __devinit cpu_attach_domain(struct sched_domain *sd, int cpu)
 
         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);
-        }
+        rcu_assign_pointer(rq->sd, sd);
 }
 
 /* cpus with isolated domains */
@@ -5112,6 +5081,7 @@ static int update_sched_domains(struct notifier_block *nfb,
         case CPU_DOWN_PREPARE:
                 for_each_online_cpu(i)
                         cpu_attach_domain(NULL, i);
+                synchronize_kernel();
                 arch_destroy_sched_domains();
                 return NOTIFY_OK;