Re: [PATCH v4] sched: fix init NOHZ_IDLE flag

[Frederic Weisbecker <fweisbec@gmail.com>]

On Wed, Feb 27, 2013 at 09:28:26AM +0100, Vincent Guittot wrote:
> > Ok I don't like having a per cpu state in struct sched domain but for
> > now I can't find anything better. So my suggestion is that we do this
> > and describe well the race, define the issue in the changelog and code
> > comments and explain how we are solving it. This way at least the
> > issue is identified and known. Then later, on review or after the
> > patch is upstream, if somebody with some good taste comes with a
> > better idea, we consider it.
> >
> > What do you think?
> 
> I don't have better solution than adding this state in the
> sched_domain if we want to keep the exact same behavior. This will be
> a bit of waste of mem because we don't need to update all sched_domain
> level (1st level is enough).

Or you can try something like the below. Both flags and sched_domain share the same
object here so the same RCU lifecycle. And there shouldn't be more overhead there
since accessing rq->sd_rq.sd is the same than rq->sd_rq in the ASM level: only
one pointer to dereference.

Also rq_idle becomes a separate value from rq->nohz_flags. It's a simple boolean
(just making it an int here because boolean size are a bit opaque, although they
are supposed to be char, let's just avoid surprises in structures).

diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index cc03cfd..16c0d55 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -417,7 +417,10 @@ struct rq {
 
 #ifdef CONFIG_SMP
 	struct root_domain *rd;
-	struct sched_domain *sd;
+	struct sched_domain_rq {
+		struct sched_domain sd;
+		int rq_idle;
+	} __rcu *sd_rq;
 
 	unsigned long cpu_power;
 
@@ -505,9 +508,14 @@ DECLARE_PER_CPU(struct rq, runqueues);
 
 #ifdef CONFIG_SMP
 
-#define rcu_dereference_check_sched_domain(p) \
-	rcu_dereference_check((p), \
-			      lockdep_is_held(&sched_domains_mutex))
+#define rcu_dereference_check_sched_domain(p) ({\
+	struct sched_domain_rq *__sd_rq = rcu_dereference_check((p),    \
+				lockdep_is_held(&sched_domains_mutex)); \
+	if (!__sd_rq)							\
+		NULL;							\
+	else								\
+		&__sd_rq->sd;						\
+})
 
 /*
  * The domain tree (rq->sd) is protected by RCU's quiescent state transition.
@@ -517,7 +525,7 @@ DECLARE_PER_CPU(struct rq, runqueues);
  * preempt-disabled sections.
  */
 #define for_each_domain(cpu, __sd) \
-	for (__sd = rcu_dereference_check_sched_domain(cpu_rq(cpu)->sd); \
+	for (__sd = rcu_dereference_check_sched_domain(cpu_rq(cpu)->sd_rq); \
 			__sd; __sd = __sd->parent)
 
 #define for_each_lower_domain(sd) for (; sd; sd = sd->child)