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Subject[RFC PATCH 3/4] Make deadline max-heap faster avoiding real swaps on heapify ops.
Date
This change achieves up to 10% of speed-up for cpudl_set() calls, as
measured with a andomly generated workload of 1K,10K,100K random heap
insertions and deletions (75% cpudl_set() calls with is_valid=1 and
25% with is_valid=0), and randomly generated cpu IDs, with up to 256
CPUs, as measured on an Intel Core2 Duo.

Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Juri Lelli <juri.lelli@arm.com>
Cc: Luca Abeni <luca.abeni@unitn.it>
Reviewed-by: Luca Abeni <luca.abeni@unitn.it>
Signed-off-by: Tommaso Cucinotta <tommaso.cucinotta@sssup.it>
---
kernel/sched/cpudeadline.c | 114 +++++++++++++++++++++++++++------------------
1 file changed, 69 insertions(+), 45 deletions(-)

diff --git a/kernel/sched/cpudeadline.c b/kernel/sched/cpudeadline.c
index 3c42702..60f933a 100644
--- a/kernel/sched/cpudeadline.c
+++ b/kernel/sched/cpudeadline.c
@@ -31,60 +31,82 @@ 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_down(struct cpudl *cp, int idx)
{
int l, r, largest;

+ int orig_cpu = cp->elements[idx].cpu;
+ u64 orig_dl = cp->elements[idx].dl;
+
+ if (left_child(idx) >= cp->size)
+ return;
+
/* 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)
{
- while (idx > 0 && dl_time_before(cp->elements[parent(idx)].dl,
- cp->elements[idx].dl)) {
- cpudl_exchange(cp, idx, parent(idx));
- idx = parent(idx);
- }
+ int p;
+
+ int orig_cpu = cp->elements[idx].cpu;
+ u64 orig_dl = cp->elements[idx].dl;
+
+ if (idx == 0)
+ return;
+
+ do {
+ 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;
+ } while (idx != 0);
+ /* 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_change_key(struct cpudl *cp, int idx, u64 new_dl)
+static void cpudl_heapify(struct cpudl *cp, int idx)
{
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_down(cp, idx);
- } else {
- cp->elements[idx].dl = new_dl;
+ 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);
}
}

@@ -153,28 +175,30 @@ 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;
- cpudl_heapify_up(cp, old_idx);
- cpumask_set_cpu(cpu, cp->free_cpus);
- cpudl_heapify_down(cp, old_idx);
-
- goto out;
- }
+ 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);
+ }

- if (old_idx == IDX_INVALID) {
- int size1 = cp->size++;
- cp->elements[size1].dl = dl;
- cp->elements[size1].cpu = cpu;
- cp->elements[cpu].idx = size1;
- cpudl_heapify_up(cp, size1);
- 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 size1 = cp->size++;
+ cp->elements[size1].dl = dl;
+ cp->elements[size1].cpu = cpu;
+ cp->elements[cpu].idx = size1;
+ cpudl_heapify_up(cp, size1);
+
+ cpumask_clear_cpu(cpu, cp->free_cpus);
+ } else {
+ cp->elements[old_idx].dl = dl;
+ cpudl_heapify(cp, old_idx);
+ }
}

out:
--
2.7.4
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