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Subject[CFS Bandwidth Control v4 6/7] sched: hierarchical task accounting for SCHED_OTHER
With task entities participating in throttled sub-trees it is possible for
task activation/de-activation to not lead to root visible changes to
rq->nr_running. This in turn leads to incorrect idle and weight-per-task load
balance decisions.

To allow correct accounting we move responsibility for updating rq->nr_running
to the respective sched::classes. In the fair-group case this update is
hierarchical, tracking the number of active tasks rooted at each group entity.

Note: technically this issue also exists with the existing sched_rt
throttling; however due to the nearly complete provisioning of system
resources for rt scheduling this is much less common by default.

Signed-off-by: Paul Turner <pjt@google.com>
Signed-off-by: Bharata B Rao <bharata@linux.vnet.ibm.com>
---
kernel/sched.c | 9 ++++++---
kernel/sched_fair.c | 42 ++++++++++++++++++++++++++++++++++++++++++
kernel/sched_rt.c | 5 ++++-
3 files changed, 52 insertions(+), 4 deletions(-)
Index: tip/kernel/sched.c
===================================================================
--- tip.orig/kernel/sched.c
+++ tip/kernel/sched.c
@@ -330,7 +330,7 @@ struct task_group root_task_group;
/* CFS-related fields in a runqueue */
struct cfs_rq {
struct load_weight load;
- unsigned long nr_running;
+ unsigned long nr_running, h_nr_tasks;

u64 exec_clock;
u64 min_vruntime;
@@ -1846,6 +1846,11 @@ static const struct sched_class rt_sched

#include "sched_stats.h"

+static void mod_nr_running(struct rq *rq, long delta)
+{
+ rq->nr_running += delta;
+}
+
static void inc_nr_running(struct rq *rq)
{
rq->nr_running++;
@@ -1896,7 +1901,6 @@ static void activate_task(struct rq *rq,
rq->nr_uninterruptible--;

enqueue_task(rq, p, flags);
- inc_nr_running(rq);
}

/*
@@ -1908,7 +1912,6 @@ static void deactivate_task(struct rq *r
rq->nr_uninterruptible++;

dequeue_task(rq, p, flags);
- dec_nr_running(rq);
}

#ifdef CONFIG_IRQ_TIME_ACCOUNTING
Index: tip/kernel/sched_fair.c
===================================================================
--- tip.orig/kernel/sched_fair.c
+++ tip/kernel/sched_fair.c
@@ -81,6 +81,8 @@ unsigned int normalized_sysctl_sched_wak

const_debug unsigned int sysctl_sched_migration_cost = 500000UL;

+static void account_hier_tasks(struct sched_entity *se, int delta);
+
/*
* The exponential sliding window over which load is averaged for shares
* distribution.
@@ -933,6 +935,40 @@ static inline void update_entity_shares_
}
#endif /* CONFIG_FAIR_GROUP_SCHED */

+#ifdef CONFIG_CFS_BANDWIDTH
+/* maintain hierarchal task counts on group entities */
+static void account_hier_tasks(struct sched_entity *se, int delta)
+{
+ struct rq *rq = rq_of(cfs_rq_of(se));
+ struct cfs_rq *cfs_rq;
+
+ for_each_sched_entity(se) {
+ /* a throttled entity cannot affect its parent hierarchy */
+ if (group_cfs_rq(se) && cfs_rq_throttled(group_cfs_rq(se)))
+ break;
+
+ /* we affect our queuing entity */
+ cfs_rq = cfs_rq_of(se);
+ cfs_rq->h_nr_tasks += delta;
+ }
+
+ /* account for global nr_running delta to hierarchy change */
+ if (!se)
+ mod_nr_running(rq, delta);
+}
+#else
+/*
+ * In the absence of group throttling, all operations are guaranteed to be
+ * globally visible at the root rq level.
+ */
+static void account_hier_tasks(struct sched_entity *se, int delta)
+{
+ struct rq *rq = rq_of(cfs_rq_of(se));
+
+ mod_nr_running(rq, delta);
+}
+#endif
+
static void enqueue_sleeper(struct cfs_rq *cfs_rq, struct sched_entity *se)
{
#ifdef CONFIG_SCHEDSTATS
@@ -1428,6 +1464,7 @@ enqueue_task_fair(struct rq *rq, struct
update_cfs_shares(cfs_rq);
}

+ account_hier_tasks(&p->se, 1);
hrtick_update(rq);
}

@@ -1461,6 +1498,7 @@ static void dequeue_task_fair(struct rq
update_cfs_shares(cfs_rq);
}

+ account_hier_tasks(&p->se, -1);
hrtick_update(rq);
}

@@ -1488,6 +1526,8 @@ static u64 tg_request_cfs_quota(struct t
return delta;
}

+static void account_hier_tasks(struct sched_entity *se, int delta);
+
static void throttle_cfs_rq(struct cfs_rq *cfs_rq)
{
struct sched_entity *se;
@@ -1507,6 +1547,7 @@ static void throttle_cfs_rq(struct cfs_r
if (!se->on_rq)
goto out_throttled;

+ account_hier_tasks(se, -cfs_rq->h_nr_tasks);
for_each_sched_entity(se) {
struct cfs_rq *cfs_rq = cfs_rq_of(se);

@@ -1541,6 +1582,7 @@ static void unthrottle_cfs_rq(struct cfs
cfs_rq->load_stamp = cfs_rq->load_last = rq->clock_task;

cfs_rq->throttled = 0;
+ account_hier_tasks(se, cfs_rq->h_nr_tasks);
for_each_sched_entity(se) {
if (se->on_rq)
break;
Index: tip/kernel/sched_rt.c
===================================================================
--- tip.orig/kernel/sched_rt.c
+++ tip/kernel/sched_rt.c
@@ -906,6 +906,8 @@ enqueue_task_rt(struct rq *rq, struct ta

if (!task_current(rq, p) && p->rt.nr_cpus_allowed > 1)
enqueue_pushable_task(rq, p);
+
+ inc_nr_running(rq);
}

static void dequeue_task_rt(struct rq *rq, struct task_struct *p, int flags)
@@ -916,6 +918,8 @@ static void dequeue_task_rt(struct rq *r
dequeue_rt_entity(rt_se);

dequeue_pushable_task(rq, p);
+
+ dec_nr_running(rq);
}

/*
@@ -1783,4 +1787,3 @@ static void print_rt_stats(struct seq_fi
rcu_read_unlock();
}
#endif /* CONFIG_SCHED_DEBUG */
-



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