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Subject[PATCH 14/15] sched: speed up -dl pushes with a push-heap.
Date
Data from tests confirmed that the original active load balancing
logic didn't scale neither in the number of CPU nor in the number of
tasks (as sched_rt does).

Here we provide a global data structure to keep track of deadlines
of the running tasks in the system. The structure is composed by
a bitmask showing the free CPUs and a max-heap, needed when the system
is heavily loaded.

The implementation and concurrent access scheme are kept simple by
design. However, our measurements show that we can compete with sched_rt
on large multi-CPUs machines [1].

Only the push path is addressed, the extension to use this structure
also for pull decisions is straightforward. However, we are currently
evaluating different (in order to decrease/avoid contention) data
structures to solve possibly both problems. We are also going to re-run
tests considering recent changes inside cpupri [2].

[1] http://retis.sssup.it/~jlelli/papers/Ospert11Lelli.pdf
[2] http://www.spinics.net/lists/linux-rt-users/msg06778.html

Signed-off-by: Juri Lelli <juri.lelli@gmail.com>
---
kernel/sched/Makefile | 2 +-
kernel/sched/core.c | 3 +
kernel/sched/cpudl.c | 208 +++++++++++++++++++++++++++++++++++++++++++++++++
kernel/sched/cpudl.h | 33 ++++++++
kernel/sched/dl.c | 52 ++++---------
kernel/sched/sched.h | 2 +
6 files changed, 260 insertions(+), 40 deletions(-)
create mode 100644 kernel/sched/cpudl.c
create mode 100644 kernel/sched/cpudl.h
diff --git a/kernel/sched/Makefile b/kernel/sched/Makefile
index 73aa1a3..fc69041 100644
--- a/kernel/sched/Makefile
+++ b/kernel/sched/Makefile
@@ -12,7 +12,7 @@ CFLAGS_core.o := $(PROFILING) -fno-omit-frame-pointer
endif

obj-y += core.o clock.o idle_task.o fair.o rt.o dl.o stop_task.o
-obj-$(CONFIG_SMP) += cpupri.o
+obj-$(CONFIG_SMP) += cpupri.o cpudl.o
obj-$(CONFIG_SCHED_AUTOGROUP) += auto_group.o
obj-$(CONFIG_SCHEDSTATS) += stats.o
obj-$(CONFIG_SCHED_DEBUG) += debug.o
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 23cfcc3..85451e9 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -6106,6 +6106,7 @@ static void free_rootdomain(struct rcu_head *rcu)
struct root_domain *rd = container_of(rcu, struct root_domain, rcu);

cpupri_cleanup(&rd->cpupri);
+ cpudl_cleanup(&rd->cpudl);
free_cpumask_var(rd->dlo_mask);
free_cpumask_var(rd->rto_mask);
free_cpumask_var(rd->online);
@@ -6164,6 +6165,8 @@ static int init_rootdomain(struct root_domain *rd)
goto free_dlo_mask;

init_dl_bw(&rd->dl_bw);
+ if (cpudl_init(&rd->cpudl) != 0)
+ goto free_dlo_mask;

if (cpupri_init(&rd->cpupri) != 0)
goto free_rto_mask;
diff --git a/kernel/sched/cpudl.c b/kernel/sched/cpudl.c
new file mode 100644
index 0000000..564017e
--- /dev/null
+++ b/kernel/sched/cpudl.c
@@ -0,0 +1,208 @@
+/*
+ * kernel/sched/cpudl.c
+ *
+ * Global CPU deadline management
+ *
+ * Author: Juri Lelli <j.lelli@sssup.it>
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; version 2
+ * of the License.
+ */
+
+#include <linux/gfp.h>
+#include <linux/kernel.h>
+#include "cpudl.h"
+
+static inline int parent(int i)
+{
+ return (i - 1) >> 1;
+}
+
+static inline int left_child(int i)
+{
+ return (i << 1) + 1;
+}
+
+static inline int right_child(int i)
+{
+ return (i << 1) + 2;
+}
+
+static inline int dl_time_before(u64 a, u64 b)
+{
+ return (s64)(a - b) < 0;
+}
+
+void cpudl_exchange(struct cpudl *cp, int a, int b)
+{
+ int cpu_a = cp->elements[a].cpu, cpu_b = cp->elements[b].cpu;
+
+ swap(cp->elements[a], cp->elements[b]);
+ swap(cp->cpu_to_idx[cpu_a], cp->cpu_to_idx[cpu_b]);
+}
+
+void cpudl_heapify(struct cpudl *cp, int idx)
+{
+ int l, r, largest;
+
+ /* adapted from lib/prio_heap.c */
+ while(1) {
+ l = left_child(idx);
+ r = right_child(idx);
+ largest = idx;
+
+ if ((l < cp->size) && dl_time_before(cp->elements[idx].dl,
+ cp->elements[l].dl))
+ largest = l;
+ if ((r < cp->size) && dl_time_before(cp->elements[largest].dl,
+ cp->elements[r].dl))
+ largest = r;
+ if (largest == idx)
+ break;
+
+ /* Push idx down the heap one level and bump one up */
+ cpudl_exchange(cp, largest, idx);
+ idx = largest;
+ }
+}
+
+void cpudl_change_key(struct cpudl *cp, int idx, u64 new_dl)
+{
+ WARN_ON(idx > num_present_cpus() && idx != -1);
+
+ if (dl_time_before(new_dl, cp->elements[idx].dl)) {
+ cp->elements[idx].dl = new_dl;
+ cpudl_heapify(cp, idx);
+ } else {
+ cp->elements[idx].dl = new_dl;
+ while (idx > 0 && dl_time_before(cp->elements[parent(idx)].dl,
+ cp->elements[idx].dl)) {
+ cpudl_exchange(cp, idx, parent(idx));
+ idx = parent(idx);
+ }
+ }
+}
+
+static inline int cpudl_maximum(struct cpudl *cp)
+{
+ return cp->elements[0].cpu;
+}
+
+/*
+ * cpudl_find - find the best (later-dl) CPU in the system
+ * @cp: the cpudl max-heap context
+ * @p: the task
+ * @later_mask: a mask to fill in with the selected CPUs (or NULL)
+ *
+ * Returns: int - best CPU (heap maximum if suitable)
+ */
+int cpudl_find(struct cpudl *cp, struct cpumask *dlo_mask,
+ struct task_struct *p, struct cpumask *later_mask)
+{
+ int best_cpu = -1;
+ const struct sched_dl_entity *dl_se = &p->dl;
+
+ if (later_mask && cpumask_and(later_mask, cp->free_cpus,
+ &p->cpus_allowed) && cpumask_and(later_mask,
+ later_mask, cpu_active_mask)) {
+ best_cpu = cpumask_any(later_mask);
+ goto out;
+ } else if (cpumask_test_cpu(cpudl_maximum(cp), &p->cpus_allowed) &&
+ dl_time_before(dl_se->deadline, cp->elements[0].dl)) {
+ best_cpu = cpudl_maximum(cp);
+ if (later_mask)
+ cpumask_set_cpu(best_cpu, later_mask);
+ }
+
+out:
+ WARN_ON(best_cpu > num_present_cpus() && best_cpu != -1);
+
+ return best_cpu;
+}
+
+/*
+ * cpudl_set - update the cpudl max-heap
+ * @cp: the cpudl max-heap context
+ * @cpu: the target cpu
+ * @dl: the new earliest deadline for this cpu
+ *
+ * Notes: assumes cpu_rq(cpu)->lock is locked
+ *
+ * Returns: (void)
+ */
+void cpudl_set(struct cpudl *cp, int cpu, u64 dl, int is_valid)
+{
+ int old_idx, new_cpu;
+ unsigned long flags;
+
+ WARN_ON(cpu > num_present_cpus());
+
+ raw_spin_lock_irqsave(&cp->lock, flags);
+ old_idx = cp->cpu_to_idx[cpu];
+ if (!is_valid) {
+ /* remove item */
+ new_cpu = cp->elements[cp->size - 1].cpu;
+ cp->elements[old_idx].dl = cp->elements[cp->size - 1].dl;
+ cp->elements[old_idx].cpu = new_cpu;
+ cp->size--;
+ cp->cpu_to_idx[new_cpu] = old_idx;
+ cp->cpu_to_idx[cpu] = IDX_INVALID;
+ while (old_idx > 0 && dl_time_before(
+ cp->elements[parent(old_idx)].dl,
+ cp->elements[old_idx].dl)) {
+ cpudl_exchange(cp, old_idx, parent(old_idx));
+ old_idx = parent(old_idx);
+ }
+ cpumask_set_cpu(cpu, cp->free_cpus);
+ cpudl_heapify(cp, old_idx);
+
+ goto out;
+ }
+
+ if (old_idx == IDX_INVALID) {
+ cp->size++;
+ cp->elements[cp->size - 1].dl = 0;
+ cp->elements[cp->size - 1].cpu = cpu;
+ cp->cpu_to_idx[cpu] = cp->size - 1;
+ cpudl_change_key(cp, cp->size - 1, dl);
+ cpumask_clear_cpu(cpu, cp->free_cpus);
+ } else {
+ cpudl_change_key(cp, old_idx, dl);
+ }
+
+out:
+ raw_spin_unlock_irqrestore(&cp->lock, flags);
+}
+
+/*
+ * cpudl_init - initialize the cpudl structure
+ * @cp: the cpudl max-heap context
+ */
+int cpudl_init(struct cpudl *cp)
+{
+ int i;
+
+ memset(cp, 0, sizeof(*cp));
+ raw_spin_lock_init(&cp->lock);
+ cp->size = 0;
+ for (i = 0; i < NR_CPUS; i++)
+ cp->cpu_to_idx[i] = IDX_INVALID;
+ if (!alloc_cpumask_var(&cp->free_cpus, GFP_KERNEL))
+ return -ENOMEM;
+ cpumask_setall(cp->free_cpus);
+
+ return 0;
+}
+
+/*
+ * cpudl_cleanup - clean up the cpudl structure
+ * @cp: the cpudl max-heap context
+ */
+void cpudl_cleanup(struct cpudl *cp)
+{
+ /*
+ * nothing to do for the moment
+ */
+}
diff --git a/kernel/sched/cpudl.h b/kernel/sched/cpudl.h
new file mode 100644
index 0000000..7f93e64
--- /dev/null
+++ b/kernel/sched/cpudl.h
@@ -0,0 +1,33 @@
+#ifndef _LINUX_CPUDL_H
+#define _LINUX_CPUDL_H
+
+#include <linux/sched.h>
+
+#define IDX_INVALID -1
+
+struct array_item {
+ u64 dl;
+ int cpu;
+};
+
+struct cpudl {
+ raw_spinlock_t lock;
+ int size;
+ int cpu_to_idx[NR_CPUS];
+ struct array_item elements[NR_CPUS];
+ cpumask_var_t free_cpus;
+};
+
+
+#ifdef CONFIG_SMP
+int cpudl_find(struct cpudl *cp, struct cpumask *dlo_mask,
+ struct task_struct *p, struct cpumask *later_mask);
+void cpudl_set(struct cpudl *cp, int cpu, u64 dl, int is_valid);
+int cpudl_init(struct cpudl *cp);
+void cpudl_cleanup(struct cpudl *cp);
+#else
+#define cpudl_set(cp, cpu, dl) do { } while (0)
+#define cpudl_init() do { } while (0)
+#endif /* CONFIG_SMP */
+
+#endif /* _LINUX_CPUDL_H */
diff --git a/kernel/sched/dl.c b/kernel/sched/dl.c
index f7f4921..941a83a 100644
--- a/kernel/sched/dl.c
+++ b/kernel/sched/dl.c
@@ -649,6 +649,7 @@ static void inc_dl_deadline(struct dl_rq *dl_rq, u64 deadline)
*/
dl_rq->earliest_dl.next = dl_rq->earliest_dl.curr;
dl_rq->earliest_dl.curr = deadline;
+ cpudl_set(&rq->rd->cpudl, rq->cpu, deadline, 1);
} else if (dl_rq->earliest_dl.next == 0 ||
dl_time_before(deadline, dl_rq->earliest_dl.next)) {
/*
@@ -672,6 +673,7 @@ static void dec_dl_deadline(struct dl_rq *dl_rq, u64 deadline)
if (!dl_rq->dl_nr_running) {
dl_rq->earliest_dl.curr = 0;
dl_rq->earliest_dl.next = 0;
+ cpudl_set(&rq->rd->cpudl, rq->cpu, 0, 0);
} else {
struct rb_node *leftmost = dl_rq->rb_leftmost;
struct sched_dl_entity *entry;
@@ -679,6 +681,7 @@ static void dec_dl_deadline(struct dl_rq *dl_rq, u64 deadline)
entry = rb_entry(leftmost, struct sched_dl_entity, rb_node);
dl_rq->earliest_dl.curr = entry->deadline;
dl_rq->earliest_dl.next = next_deadline(rq);
+ cpudl_set(&rq->rd->cpudl, rq->cpu, entry->deadline, 1);
}
}

@@ -856,9 +859,6 @@ static void yield_task_dl(struct rq *rq)
#ifdef CONFIG_SMP

static int find_later_rq(struct task_struct *task);
-static int latest_cpu_find(struct cpumask *span,
- struct task_struct *task,
- struct cpumask *later_mask);

static int
select_task_rq_dl(struct task_struct *p, int sd_flag, int flags)
@@ -908,7 +908,7 @@ static void check_preempt_equal_dl(struct rq *rq, struct task_struct *p)
* let's hope p can move out.
*/
if (rq->curr->dl.nr_cpus_allowed == 1 ||
- latest_cpu_find(rq->rd->span, rq->curr, NULL) == -1)
+ cpudl_find(&rq->rd->cpudl, rq->rd->dlo_mask, rq->curr, NULL) == -1)
return;

/*
@@ -916,7 +916,7 @@ static void check_preempt_equal_dl(struct rq *rq, struct task_struct *p)
* see if it is pushed or pulled somewhere else.
*/
if (p->dl.nr_cpus_allowed != 1 &&
- latest_cpu_find(rq->rd->span, p, NULL) != -1)
+ cpudl_find(&rq->rd->cpudl, rq->rd->dlo_mask, p, NULL) != -1)
return;

resched_task(rq->curr);
@@ -1109,39 +1109,6 @@ next_node:
return NULL;
}

-static int latest_cpu_find(struct cpumask *span,
- struct task_struct *task,
- struct cpumask *later_mask)
-{
- const struct sched_dl_entity *dl_se = &task->dl;
- int cpu, found = -1, best = 0;
- u64 max_dl = 0;
-
- for_each_cpu(cpu, span) {
- struct rq *rq = cpu_rq(cpu);
- struct dl_rq *dl_rq = &rq->dl;
-
- if (cpumask_test_cpu(cpu, &task->cpus_allowed) &&
- (!dl_rq->dl_nr_running || dl_time_before(dl_se->deadline,
- dl_rq->earliest_dl.curr))) {
- if (later_mask)
- cpumask_set_cpu(cpu, later_mask);
- if (!best && !dl_rq->dl_nr_running) {
- best = 1;
- found = cpu;
- } else if (!best &&
- dl_time_before(max_dl,
- dl_rq->earliest_dl.curr)) {
- max_dl = dl_rq->earliest_dl.curr;
- found = cpu;
- }
- } else if (later_mask)
- cpumask_clear_cpu(cpu, later_mask);
- }
-
- return found;
-}
-
static DEFINE_PER_CPU(cpumask_var_t, local_cpu_mask_dl);

static int find_later_rq(struct task_struct *task)
@@ -1158,7 +1125,9 @@ static int find_later_rq(struct task_struct *task)
if (task->dl.nr_cpus_allowed == 1)
return -1;

- best_cpu = latest_cpu_find(task_rq(task)->rd->span, task, later_mask);
+ best_cpu = cpudl_find(&task_rq(task)->rd->cpudl,
+ task_rq(task)->rd->dlo_mask,
+ task, later_mask);
if (best_cpu == -1)
return -1;

@@ -1524,6 +1493,9 @@ static void rq_online_dl(struct rq *rq)
{
if (rq->dl.overloaded)
dl_set_overload(rq);
+
+ if (rq->dl.dl_nr_running > 0)
+ cpudl_set(&rq->rd->cpudl, rq->cpu, rq->dl.earliest_dl.curr, 1);
}

/* Assumes rq->lock is held */
@@ -1531,6 +1503,8 @@ static void rq_offline_dl(struct rq *rq)
{
if (rq->dl.overloaded)
dl_clear_overload(rq);
+
+ cpudl_set(&rq->rd->cpudl, rq->cpu, 0, 0);
}

void init_sched_dl_class(void)
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index d32b7b6..6e32ddd 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -5,6 +5,7 @@
#include <linux/stop_machine.h>

#include "cpupri.h"
+#include "cpudl.h"

extern __read_mostly int scheduler_running;

@@ -444,6 +445,7 @@ struct root_domain {
cpumask_var_t dlo_mask;
atomic_t dlo_count;
struct dl_bw dl_bw;
+ struct cpudl cpudl;

/*
* The "RT overload" flag: it gets set if a CPU has more than
--
1.7.9.5


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