Re: [PATCH Resend v5] sched: fix init NOHZ_IDLE flag

On Wed, Apr 03, 2013 at 03:37:43PM +0200, Vincent Guittot wrote:

On my smp platform which is made of 5 cores in 2 clusters, I have the nr_busy_cpu field of sched_group_power struct that is not null when the platform is fully idle. The root cause is: During the boot sequence, some CPUs reach the idle loop and set their NOHZ_IDLE flag while waiting for others CPUs to boot. But the nr_busy_cpus field is initialized later with the assumption that all CPUs are in the busy state whereas some CPUs have already set their NOHZ_IDLE flag.

More generally, the NOHZ_IDLE flag must be initialized when new sched_domains are created in order to ensure that NOHZ_IDLE and nr_busy_cpus are aligned.

This condition can be ensured by adding a synchronize_rcu between the destruction of old sched_domains and the creation of new ones so the NOHZ_IDLE flag will not be updated with old sched_domain once it has been initialized. But this solution introduces a additionnal latency in the rebuild sequence that is called during cpu hotplug.

As suggested by Frederic Weisbecker, another solution is to have the same rcu lifecycle for both NOHZ_IDLE and sched_domain struct. I have introduce a new sched_domain_rq struct that is the entry point for both sched_domains and objects that must follow the same lifecycle like NOHZ_IDLE flags. They will share the same RCU lifecycle and will be always synchronized.

The synchronization is done at the cost of :

• an additional indirection for accessing the first sched_domain level
• an additional indirection and a rcu_dereference before accessing to the NOHZ_IDLE flag.

Change since v4:

• link both sched_domain and NOHZ_IDLE flag in one RCU object so their states are always synchronized.

Change since V3;

• NOHZ flag is not cleared if a NULL domain is attached to the CPU
• Remove patch 2/2 which becomes useless with latest modifications

Change since V2:

• change the initialization to idle state instead of busy state so a CPU that enters idle during the build of the sched_domain will not corrupt the initialization state

Change since V1:

• remove the patch for SCHED softirq on an idle core use case as it was a side effect of the other use cases.

Signed-off-by: Vincent Guittot vincent.guittot@linaro.org

Ok, the lockless scheme involving nr_buzy_cpus and rq flags seem to be correct now. We can hope somebody will come up with a less complicated solution. But for now that's the best fix I've seen.

I just have a few comments on details.

---

include/linux/sched.h | 6 +++ kernel/sched/core.c | 105 ++++++++++++++++++++++++++++++++++++++++++++----- kernel/sched/fair.c | 35 +++++++++++------ kernel/sched/sched.h | 24 +++++++++-- 4 files changed, 145 insertions(+), 25 deletions(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h index d35d2b6..2a52188 100644 --- a/include/linux/sched.h +++ b/include/linux/sched.h @@ -959,6 +959,12 @@ struct sched_domain { unsigned long span[0]; };

+struct sched_domain_rq {

• struct sched_domain *sd;
• unsigned long flags;
• struct rcu_head rcu; /* used during destruction */

+};

So the reason for this level of indirection won't be intuitive for those who read that code. Please add some comments that explain why we need that. ie: because we need the object lifecycle of sched_power and flags to be the same for the lockless scheme to work.

static inline struct cpumask *sched_domain_span(struct sched_domain *sd) { return to_cpumask(sd->span); diff --git a/kernel/sched/core.c b/kernel/sched/core.c index 7f12624..69e2313 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -5602,6 +5602,15 @@ static void destroy_sched_domains(struct sched_domain *sd, int cpu) destroy_sched_domain(sd, cpu); } +static void destroy_sched_domain_rq(struct sched_domain_rq *sd_rq, int cpu) +{

• if (!sd_rq)

• return;
  

• destroy_sched_domains(sd_rq->sd, cpu);
• kfree_rcu(sd_rq, rcu);

+}

/*

• Keep a special pointer to the highest sched_domain that has
• SD_SHARE_PKG_RESOURCE set (Last Level Cache Domain) for this

@@ -5632,10 +5641,23 @@ static void update_top_cache_domain(int cpu)

• hold the hotplug lock.

*/ static void -cpu_attach_domain(struct sched_domain *sd, struct root_domain *rd, int cpu) +cpu_attach_domain(struct sched_domain_rq *sd_rq, struct root_domain *rd,

• int cpu)
  

{ struct rq *rq = cpu_rq(cpu);

• struct sched_domain *tmp;

• struct sched_domain_rq *tmp_rq;

old_sd_rq would be a clearer name.

• struct sched_domain *tmp, *sd = NULL;
• /*

• * If we don't have any sched_domain and associated object, we can
  

• * directly jump to the attach sequence otherwise we try to degenerate
  

• * the sched_domain
  

• */
  

• if (!sd_rq)

• goto attach;
  

• /* Get a pointer to the 1st sched_domain */
• sd = sd_rq->sd;

/* Remove the sched domains which do not contribute to scheduling. */ for (tmp = sd; tmp; ) { @@ -5658,14 +5680,17 @@ cpu_attach_domain(struct sched_domain *sd, struct root_domain *rd, int cpu) destroy_sched_domain(tmp, cpu); if (sd) sd->child = NULL;

• /* update sched_domain_rq */
  

• sd_rq->sd = sd;
  

  }

+attach: sched_domain_debug(sd, cpu); rq_attach_root(rq, rd);

• tmp = rq->sd;
• rcu_assign_pointer(rq->sd, sd);
• destroy_sched_domains(tmp, cpu);

• tmp_rq = rq->sd_rq;
• rcu_assign_pointer(rq->sd_rq, sd_rq);
• destroy_sched_domain_rq(tmp_rq, cpu);

update_top_cache_domain(cpu);

[...]

+static void __sdrq_free(const struct cpumask *cpu_map, struct s_data *d) +{

• int j;
• for_each_cpu(j, cpu_map)

• if (*per_cpu_ptr(d->sd_rq, j))
  

• 	kfree(*per_cpu_ptr(d->sd_rq, j));
  

kfree(NULL) works.

+}

+static void build_sched_domain_rq(struct s_data *d, int cpu) +{

• struct sched_domain_rq *sd_rq;
• struct sched_domain *sd;
• /* Attach sched_domain to sched_domain_rq */
• sd = *per_cpu_ptr(d->sd, cpu);
• sd_rq = *per_cpu_ptr(d->sd_rq, cpu);
• sd_rq->sd = sd;
• /* Init flags */
• set_bit(NOHZ_IDLE, sched_rq_flags(sd_rq));

+}

struct sched_domain *build_sched_domain(struct sched_domain_topology_level *tl, struct s_data *d, const struct cpumask *cpu_map, struct sched_domain_attr *attr, struct sched_domain *child, @@ -6495,6 +6572,7 @@ static int build_sched_domains(const struct cpumask *cpu_map, struct sched_domain_attr *attr) { enum s_alloc alloc_state = sa_none;

• struct sched_domain_rq *sd_rq; struct sched_domain *sd; struct s_data d; int i, ret = -ENOMEM;

@@ -6547,11 +6625,18 @@ static int build_sched_domains(const struct cpumask *cpu_map, } }

• /* Init objects that must follow the sched_domain lifecycle */
• for_each_cpu(i, cpu_map) {

• build_sched_domain_rq(&d, i);
  

• }

I suggest you put the above domain_rq initialization before the domain initialization.

So that we have that more intuitively ordered initialization:

* set up domains_rq * set up domains * set up sched groups * set up sched groups power

• /* Attach the domains */ rcu_read_lock(); for_each_cpu(i, cpu_map) {

• sd = *per_cpu_ptr(d.sd, i);
  

• cpu_attach_domain(sd, d.rd, i);
  

• sd_rq = *per_cpu_ptr(d.sd_rq, i);
  

• cpu_attach_domain(sd_rq, d.rd, i);
  

• /* claim allocation of sched_domain_rq object */
  

• *per_cpu_ptr(d.sd_rq, i) = NULL;
  

  } rcu_read_unlock();

[...]

@@ -5673,7 +5681,12 @@ static void run_rebalance_domains(struct softirq_action *h) static inline int on_null_domain(int cpu) {

• return !rcu_dereference_sched(cpu_rq(cpu)->sd);

• struct sched_domain_rq *sd_rq =

• rcu_dereference_sched(cpu_rq(cpu)->sd_rq);
  

• struct sched_domain *sd = NULL;
• if (sd_rq)

• sd = sd_rq->sd;
  

Is it possible to have sd_rq->sd == NULL ?

• return !sd;

} /* diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h index cc03cfd..f589306 100644 --- a/kernel/sched/sched.h +++ b/kernel/sched/sched.h @@ -417,7 +417,7 @@ struct rq { #ifdef CONFIG_SMP struct root_domain *rd;

• struct sched_domain *sd;

• struct sched_domain_rq *sd_rq;

unsigned long cpu_power; @@ -505,21 +505,37 @@ DECLARE_PER_CPU(struct rq, runqueues); #ifdef CONFIG_SMP -#define rcu_dereference_check_sched_domain(p) \ +#define rcu_dereference_check_sched_domain_rq(p) \ rcu_dereference_check((p), \ lockdep_is_held(&sched_domains_mutex)) +#define get_sched_domain_rq(cpu) \

• rcu_dereference_check_sched_domain_rq(cpu_rq(cpu)->sd_rq)

How about rcu_dereference_domain_rq()? It seems important to me that we keep the rcu_dereference_*() naming so that we don't hide what really happens there behind a more opaque naming.

I mean RCU is tricky to deal with and it's important not to obfuscate its use.

+#define rcu_dereference_check_sched_domain(cpu) ({ \

• struct sched_domain_rq *__sd_rq = get_sched_domain_rq(cpu); \
• struct sched_domain *__sd = NULL; \
• if (__sd_rq) \

• __sd = __sd_rq->sd; \
  

Same question for the NULL sd.

• __sd; \

+})

+#define sched_rq_flags(sd_rq) (&sd_rq->flags)

Hm, why this "sched_" prefix? Maybe rq_domain_flags() ?

Thanks.