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

On 11 April 2013 16:56, Frederic Weisbecker fweisbec@gmail.com wrote:

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.

Thanks

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.

ok, i will add a comment

...

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.

ok

...

• 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.

thanks

...

+}

+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.

I can't because build_sched_domain will init struct s_data d that is used in build_sched_domain_rq

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 ?

I think so with the isolcpus as an example

...

• 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.

you're right

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() ?

ok i'm going to change the name

Thanks.