sched/deadline: speed up SCHED_DEADLINE pushes with a push-heap

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>
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1383831828-15501-14-git-send-email-juri.lelli@gmail.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

kernel/sched/Makefile

@@ -14,7 +14,7 @@ endif
 obj-y += core.o proc.o clock.o cputime.o
 obj-y += idle_task.o fair.o rt.o deadline.o stop_task.o
 obj-y += wait.o completion.o
-obj-$(CONFIG_SMP) += cpupri.o
+obj-$(CONFIG_SMP) += cpupri.o cpudeadline.o
 obj-$(CONFIG_SCHED_AUTOGROUP) += auto_group.o
 obj-$(CONFIG_SCHEDSTATS) += stats.o
 obj-$(CONFIG_SCHED_DEBUG) += debug.o

kernel/sched/core.c

@@ -5287,6 +5287,7 @@ 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);
@@ -5345,6 +5346,8 @@ 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;

kernel/sched/cpudeadline.c

@@ -0,0 +1,216 @@
+/*
+ *  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 "cpudeadline.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 == IDX_INVALID);
+
+	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 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 */
+		if (old_idx == IDX_INVALID) {
+			/*
+			 * Nothing to remove if old_idx was invalid.
+			 * This could happen if a rq_offline_dl is
+			 * called for a CPU without -dl tasks running.
+			 */
+			goto out;
+		}
+		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
+	 */
+}

kernel/sched/cpudeadline.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 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 */