[RFC PATCH 2/3] sched/deadline: make cpu heap faster avoiding real swaps on heapify

Tommaso Cucinotta
2016-08-14T14:27:47Z

This change goes from heapify() ops done by swapping with parent/child
so that the item to fix moves along, to heapify() ops done by just
pulling the parent/child chain by 1 pos, then storing the item to fix
just at the end. On a non-trivial heapify(), this performs roughly half
stores wrt swaps.

This has been measured to achieve up to 10% of speed-up for cpudl_set()
calls, with a randomly 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>
Reviewed-by: Juri Lelli <juri.lelli@arm.com>
Signed-off-by: Tommaso Cucinotta <tommaso.cucinotta@sssup.it>
---
kernel/sched/cpudeadline.c | 66 +++++++++++++++++++++++++++++++---------------
1 file changed, 45 insertions(+), 21 deletions(-)

diff --git a/kernel/sched/cpudeadline.c b/kernel/sched/cpudeadline.c
index 0acb0d4..0ace75a 100644
--- a/kernel/sched/cpudeadline.c
+++ b/kernel/sched/cpudeadline.c
@@ -31,48 +31,72 @@ 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(orig_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_heapify(struct cpudl *cp, int idx)
--
2.7.4