SCHED_DEADLINE cpudeadline.{h,c} fixup

[Tommaso Cucinotta <tommaso.cucinotta@sssup.it>]

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Hi,

looking at the SCHED_DEADLINE code, I spotted an opportunity to
make cpudeadline.c faster, in that we can skip real swaps during
re-heapify()ication of items after addition/removal. As such ops
are done under a domain spinlock, it sounded like an interesting
try.

Indeed, I've got a speed-up of up to ~6% for the cpudl_set() calls
on a randomly generated workload of 1K,10K,100K random insertions
and deletions (75% cpudl_set() calls with is_valid=1 and 25% with
is_valid=0), and randomly generated cpu IDs with 2, 4, ..., 256 CPUs.
Details in the attached plot.

The attached patch does this, along with a minimum of rework of
cpudeadline.c internals, and a final clean-up of the cpudeadline.h
interface (second patch).

The measurements have been made on an Intel Core2 Duo with the CPU
frequency fixed at max, by letting cpudeadline.c be initialized with
various numbers of CPUs, then  making many calls sequentially, taking
the rdtsc among calls, then dumping all numbers through printk(),
and I'm plotting the average of clock ticks between consecutive calls.
[ I can share the benchmarking code as well if needed ]

Also, this fixes what seems to me a bug I noticed comparing the whole
heap contents as handledbut the modified code vs the original one,
insertion by insertion. The problem is in this code:

		cp->elements[cp->size - 1].dl = 0;
		cp->elements[cp->size - 1].cpu = cpu;
		cp->elements[cpu].idx = cp->size - 1;
		mycpudl_change_key(cp, cp->size - 1, dl);

when fed by an absolute deadline that is so large to have a negative
value as a s64. In such a case, as from dl_time_before(), the kernel
should handle correctly the abs deadline wrap-around, however the
current code in cpudeadline.c goes mad, and doesn't re-heapify
correctly the just inserted element... that said, if these are ns,
such a bug should be hit after a ~292 years of uptime :-D...

I'd be happy to hear comments from others. I can provide additional
info / make additional experiments as needed.

Please, reply-all to this e-mail, I'm not subscribed to linux-kernel@.

Thanks,

	Tommaso
-- 
Tommaso Cucinotta, Computer Engineering PhD
Associate Professor at the Real-Time Systems Laboratory (ReTiS)
Scuola Superiore Sant'Anna, Pisa, Italy
http://retis.sssup.it/people/tommaso

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>From ee54c2849f1d9d7f7f8faeb474a61074cae868b9 Mon Sep 17 00:00:00 2001
From: Tommaso Cucinotta <tommaso@lyx.org>
Date: Thu, 12 May 2016 19:06:37 +0200
Subject: [PATCH 1/2] Make deadline max-heap faster and fix deadline
 wrap-around bug.

---
 kernel/sched/cpudeadline.c | 122 ++++++++++++++++++++++++++++-----------------
 1 file changed, 77 insertions(+), 45 deletions(-)

diff --git a/kernel/sched/cpudeadline.c b/kernel/sched/cpudeadline.c
index 5a75b08..245d929 100644
--- a/kernel/sched/cpudeadline.c
+++ b/kernel/sched/cpudeadline.c
@@ -31,55 +31,91 @@ static inline int right_child(int i)
 	return (i << 1) + 2;
 }
 
-static 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].cpu, cp->elements[b].cpu);
-	swap(cp->elements[a].dl , cp->elements[b].dl );
-
-	swap(cp->elements[cpu_a].idx, cp->elements[cpu_b].idx);
-}
-
-static void cpudl_heapify(struct cpudl *cp, int idx)
+static void cpudl_heapify_down(struct cpudl *cp, int idx)
 {
 	int l, r, largest;
 
+	int orig_cpu = cp->elements[idx].cpu;
+	u64 orig_dl = cp->elements[idx].dl;
+
 	/* adapted from lib/prio_heap.c */
 	while(1) {
+		u64 largest_dl;
 		l = left_child(idx);
 		r = right_child(idx);
 		largest = idx;
+		largest_dl = orig_dl;
 
-		if ((l < cp->size) && dl_time_before(cp->elements[idx].dl,
-							cp->elements[l].dl))
+		if ((l < cp->size) && dl_time_before(orig_dl, cp->elements[l].dl)) {
 			largest = l;
-		if ((r < cp->size) && dl_time_before(cp->elements[largest].dl,
-							cp->elements[r].dl))
+			largest_dl = cp->elements[l].dl;
+		}
+		if ((r < cp->size) && dl_time_before(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);
+		/* pull largest child onto idx */
+		cp->elements[idx].cpu = cp->elements[largest].cpu;
+		cp->elements[idx].dl = cp->elements[largest].dl;
+		cp->elements[cp->elements[idx].cpu].idx = idx;
 		idx = largest;
 	}
+	/* actual push down of saved original values orig_* */
+	cp->elements[idx].cpu = orig_cpu;
+	cp->elements[idx].dl = orig_dl;
+	cp->elements[cp->elements[idx].cpu].idx = idx;
+}
+
+static void cpudl_heapify_up(struct cpudl *cp, int idx)
+{
+	int p;
+
+	int orig_cpu = cp->elements[idx].cpu;
+	u64 orig_dl = cp->elements[idx].dl;
+
+	while (idx != 0) {
+		p = parent(idx);
+		if (dl_time_before(cp->elements[idx].dl, cp->elements[p].dl))
+			break;
+		/* pull parent onto idx */
+		cp->elements[idx].cpu = cp->elements[p].cpu;
+		cp->elements[idx].dl = cp->elements[p].dl;
+		cp->elements[cp->elements[idx].cpu].idx = idx;
+		idx = p;
+	}
+	/* actual push up of saved original values orig_* */
+	cp->elements[idx].cpu = orig_cpu;
+	cp->elements[idx].dl = orig_dl;
+	cp->elements[cp->elements[idx].cpu].idx = idx;
+}
+
+static void cpudl_heapify(struct cpudl *cp, int idx)
+{
+	WARN_ON(idx == IDX_INVALID || !cpu_present(idx));
+	if (idx == IDX_INVALID)
+		return;
+
+	if (idx > 0 && dl_time_before(cp->elements[parent(idx)].dl, cp->elements[idx].dl)) {
+		cpudl_heapify_up(cp, idx);
+	} else {
+		cpudl_heapify_down(cp, idx);
+	}
 }
 
 static void cpudl_change_key(struct cpudl *cp, int idx, u64 new_dl)
 {
 	WARN_ON(idx == IDX_INVALID || !cpu_present(idx));
+	if (idx == IDX_INVALID)
+	  return;
 
 	if (dl_time_before(new_dl, cp->elements[idx].dl)) {
 		cp->elements[idx].dl = new_dl;
-		cpudl_heapify(cp, idx);
+		cpudl_heapify_down(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);
-		}
+		cpudl_heapify_up(cp, idx);
 	}
 }
 
@@ -148,33 +184,29 @@ void cpudl_set(struct cpudl *cp, int cpu, u64 dl, int is_valid)
 			 */
 			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->elements[new_cpu].idx = old_idx;
 		cp->elements[cpu].idx = 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);
+		if (old_idx != cp->size) {
+			new_cpu = cp->elements[cp->size].cpu;
+			cp->elements[old_idx].dl = cp->elements[cp->size].dl;
+			cp->elements[old_idx].cpu = new_cpu;
+			cp->elements[new_cpu].idx = old_idx;
+			cpudl_heapify(cp, 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->elements[cpu].idx = cp->size - 1;
-		cpudl_change_key(cp, cp->size - 1, dl);
-		cpumask_clear_cpu(cpu, cp->free_cpus);
+		cpumask_set_cpu(cpu, cp->free_cpus);
 	} else {
-		cpudl_change_key(cp, old_idx, dl);
+		if (old_idx == IDX_INVALID) {
+			int sz1 = cp->size++;
+			cp->elements[sz1].dl = dl;
+			cp->elements[sz1].cpu = cpu;
+			cp->elements[cpu].idx = sz1;
+			cpudl_heapify_up(cp, sz1);
+
+			cpumask_clear_cpu(cpu, cp->free_cpus);
+		} else {
+			cpudl_change_key(cp, old_idx, dl);
+		}
 	}
 
 out:
-- 
2.7.4


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>From 903d4a0ea0df831e62fc8016ce55a77939d52dbc Mon Sep 17 00:00:00 2001
From: Tommaso Cucinotta <tommaso@lyx.org>
Date: Fri, 13 May 2016 11:47:36 +0200
Subject: [PATCH 2/2] Split cpudl_set() into cpudl_set() and cpudl_clear().
 These 2 exercise independent code paths and need different arguments.

---
 kernel/sched/cpudeadline.c | 69 +++++++++++++++++++++++++++++-----------------
 kernel/sched/cpudeadline.h |  3 +-
 kernel/sched/deadline.c    | 10 +++----
 3 files changed, 51 insertions(+), 31 deletions(-)

diff --git a/kernel/sched/cpudeadline.c b/kernel/sched/cpudeadline.c
index 245d929..82e7c66 100644
--- a/kernel/sched/cpudeadline.c
+++ b/kernel/sched/cpudeadline.c
@@ -156,16 +156,15 @@ out:
 }
 
 /*
- * cpudl_set - update the cpudl max-heap
+ * cpudl_clear - remove a cpu from 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)
+void cpudl_clear(struct cpudl *cp, int cpu)
 {
 	int old_idx, new_cpu;
 	unsigned long flags;
@@ -173,17 +172,16 @@ void cpudl_set(struct cpudl *cp, int cpu, u64 dl, int is_valid)
 	WARN_ON(!cpu_present(cpu));
 
 	raw_spin_lock_irqsave(&cp->lock, flags);
+
 	old_idx = cp->elements[cpu].idx;
-	if (!is_valid) {
+	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.
+		 */
+	} else {
 		/* 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;
-		}
 		cp->size--;
 		cp->elements[cpu].idx = IDX_INVALID;
 		if (old_idx != cp->size) {
@@ -193,23 +191,44 @@ void cpudl_set(struct cpudl *cp, int cpu, u64 dl, int is_valid)
 			cp->elements[new_cpu].idx = old_idx;
 			cpudl_heapify(cp, old_idx);
 		}
-
 		cpumask_set_cpu(cpu, cp->free_cpus);
+	}
+
+	raw_spin_unlock_irqrestore(&cp->lock, flags);
+}
+
+/*
+ * 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 old_idx;
+	unsigned long flags;
+
+	WARN_ON(!cpu_present(cpu));
+
+	raw_spin_lock_irqsave(&cp->lock, flags);
+
+	old_idx = cp->elements[cpu].idx;
+	if (old_idx == IDX_INVALID) {
+		int sz1 = cp->size++;
+		cp->elements[sz1].dl = dl;
+		cp->elements[sz1].cpu = cpu;
+		cp->elements[cpu].idx = sz1;
+		cpudl_heapify_up(cp, sz1);
+
+		cpumask_clear_cpu(cpu, cp->free_cpus);
 	} else {
-		if (old_idx == IDX_INVALID) {
-			int sz1 = cp->size++;
-			cp->elements[sz1].dl = dl;
-			cp->elements[sz1].cpu = cpu;
-			cp->elements[cpu].idx = sz1;
-			cpudl_heapify_up(cp, sz1);
-
-			cpumask_clear_cpu(cpu, cp->free_cpus);
-		} else {
-			cpudl_change_key(cp, old_idx, dl);
-		}
+		cpudl_change_key(cp, old_idx, dl);
 	}
 
-out:
 	raw_spin_unlock_irqrestore(&cp->lock, flags);
 }
 
diff --git a/kernel/sched/cpudeadline.h b/kernel/sched/cpudeadline.h
index fcbdf83..f7da8c5 100644
--- a/kernel/sched/cpudeadline.h
+++ b/kernel/sched/cpudeadline.h
@@ -23,7 +23,8 @@ struct cpudl {
 #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);
+void cpudl_set(struct cpudl *cp, int cpu, u64 dl);
+void cpudl_clear(struct cpudl *cp, int cpu);
 int cpudl_init(struct cpudl *cp);
 void cpudl_set_freecpu(struct cpudl *cp, int cpu);
 void cpudl_clear_freecpu(struct cpudl *cp, int cpu);
diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index 686ec8a..e3ffc2f 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -795,7 +795,7 @@ static void inc_dl_deadline(struct dl_rq *dl_rq, u64 deadline)
 	if (dl_rq->earliest_dl.curr == 0 ||
 	    dl_time_before(deadline, dl_rq->earliest_dl.curr)) {
 		dl_rq->earliest_dl.curr = deadline;
-		cpudl_set(&rq->rd->cpudl, rq->cpu, deadline, 1);
+		cpudl_set(&rq->rd->cpudl, rq->cpu, deadline);
 	}
 }
 
@@ -810,14 +810,14 @@ 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);
+		cpudl_clear(&rq->rd->cpudl, rq->cpu);
 	} else {
 		struct rb_node *leftmost = dl_rq->rb_leftmost;
 		struct sched_dl_entity *entry;
 
 		entry = rb_entry(leftmost, struct sched_dl_entity, rb_node);
 		dl_rq->earliest_dl.curr = entry->deadline;
-		cpudl_set(&rq->rd->cpudl, rq->cpu, entry->deadline, 1);
+		cpudl_set(&rq->rd->cpudl, rq->cpu, entry->deadline);
 	}
 }
 
@@ -1667,7 +1667,7 @@ static void rq_online_dl(struct rq *rq)
 
 	cpudl_set_freecpu(&rq->rd->cpudl, rq->cpu);
 	if (rq->dl.dl_nr_running > 0)
-		cpudl_set(&rq->rd->cpudl, rq->cpu, rq->dl.earliest_dl.curr, 1);
+		cpudl_set(&rq->rd->cpudl, rq->cpu, rq->dl.earliest_dl.curr);
 }
 
 /* Assumes rq->lock is held */
@@ -1676,7 +1676,7 @@ 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);
+	cpudl_clear(&rq->rd->cpudl, rq->cpu);
 	cpudl_clear_freecpu(&rq->rd->cpudl, rq->cpu);
 }
 
-- 
2.7.4


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