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

From: Frederic Weisbecker
Date: Wed Feb 27 2013 - 11:13:40 EST

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-> 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 {

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);


-#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)

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