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Date: Tue, 2 Aug 2016 12:38:44 +0100
From: Juri Lelli <juri.lelli@....com>
To: Tommaso Cucinotta <tommaso.cucinotta@...up.it>
Cc: Luca Abeni <luca.abeni@...tn.it>,
Juri Lelli <juri.lelli@...il.com>,
Peter Zijlstra <peterz@...radead.org>,
Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 3/4] Make deadline max-heap faster avoiding real
swaps on heapify ops.
Hi,
On 19/07/16 11:44, Tommaso Cucinotta wrote:
> 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@...radead.org>
> Cc: Juri Lelli <juri.lelli@....com>
> Cc: Luca Abeni <luca.abeni@...tn.it>
> Reviewed-by: Luca Abeni <luca.abeni@...tn.it>
> Signed-off-by: Tommaso Cucinotta <tommaso.cucinotta@...up.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;
OK, I was a bit puzzled by the usage of largest_dl in this loop, but I
seemed to convince myself that it is right. It's basically a local tmp
variable that you use to pick left or right child at each iteration, and
you reset it to the deadline of the node your are moving (orig_dl) after
it iteration.
> + }
> + if ((r < cp->size) && dl_time_before(largest_dl, cp->elements[r].dl))
Nitpick, this and the above conditions break 80 columns.
> 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;
Can we actually get here with IDX_INVALID? We bail out in the !is_valid
branch and the other call point is an else branch of old_idx ==
IDX_INVALID. So, it seems that this is more a BUG_ON().
>
> - 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)) {
You can wrap after the comma. And brackets are not required.
> 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:
> --
Couldn't spot any problem with this change (apart from the minor things
I pointed out above). Didn't test it extensively, though. But it seems
you did. :)
Best,
- Juri
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