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SubjectRe: [PATCH] sched/fair: Rearrange select_task_rq_fair() to optimize it
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Hi,

On 23/04/18 11:38, Viresh Kumar wrote:
> Rearrange select_task_rq_fair() a bit to avoid executing some
> conditional statements in few specific code-paths. That gets rid of the
> goto as well.
>

I'd argue making things easier to read is a non-negligible part as well.

> This shouldn't result in any functional changes.
>
> Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
> ---
> kernel/sched/fair.c | 24 +++++++++---------------
> 1 file changed, 9 insertions(+), 15 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 54dc31e7ab9b..cacee15076a4 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -6636,6 +6636,7 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int sd_flag, int wake_f
> */
> if (want_affine && (tmp->flags & SD_WAKE_AFFINE) &&
> cpumask_test_cpu(prev_cpu, sched_domain_span(tmp))) {
> + sd = NULL; /* Prefer wake_affine over balance flags */
> affine_sd = tmp;
> break;
> }
> @@ -6646,33 +6647,26 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int sd_flag, int wake_f
> break;
> }
>
> - if (affine_sd) {
> - sd = NULL; /* Prefer wake_affine over balance flags */
> - if (cpu == prev_cpu)
> - goto pick_cpu;
> -
> - new_cpu = wake_affine(affine_sd, p, cpu, prev_cpu, sync);
> - }
> -
> - if (sd && !(sd_flag & SD_BALANCE_FORK)) {
> + if (sd) {
> /*
> * We're going to need the task's util for capacity_spare_wake
> * in find_idlest_group. Sync it up to prev_cpu's
> * last_update_time.
> */
> - sync_entity_load_avg(&p->se);
> - }
> + if (!(sd_flag & SD_BALANCE_FORK))
> + sync_entity_load_avg(&p->se);
> +
> + new_cpu = find_idlest_cpu(sd, p, cpu, prev_cpu, sd_flag);
> + } else {
> + if (affine_sd && cpu != prev_cpu)
> + new_cpu = wake_affine(affine_sd, p, cpu, prev_cpu, sync);
>
> - if (!sd) {
> -pick_cpu:
> if (sd_flag & SD_BALANCE_WAKE) { /* XXX always ? */
> new_cpu = select_idle_sibling(p, prev_cpu, new_cpu);
>
> if (want_affine)
> current->recent_used_cpu = cpu;
> }
> - } else {
> - new_cpu = find_idlest_cpu(sd, p, cpu, prev_cpu, sd_flag);
> }
> rcu_read_unlock();
>
>

I stared at it for a bit and don't see anything wrong. I was just thinking
that the original flow made it a bit clearer to follow the 'wake_affine' path.

What about this ? It re-bumps up the number of conditionals and adds an indent
level in the domain loop (that could be prevented by hiding the
cpu != prev_cpu check in wake_affine()), which isn't great, but you get to
squash some more if's.

---------------------->8-------------------------

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index cacee15..ad09b67 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6613,7 +6613,7 @@ static int wake_cap(struct task_struct *p, int cpu, int prev_cpu)
static int
select_task_rq_fair(struct task_struct *p, int prev_cpu, int sd_flag, int wake_flags)
{
- struct sched_domain *tmp, *affine_sd = NULL, *sd = NULL;
+ struct sched_domain *tmp, *sd = NULL;
int cpu = smp_processor_id();
int new_cpu = prev_cpu;
int want_affine = 0;
@@ -6636,8 +6636,10 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int sd_flag, int wake_f
*/
if (want_affine && (tmp->flags & SD_WAKE_AFFINE) &&
cpumask_test_cpu(prev_cpu, sched_domain_span(tmp))) {
+ if (cpu != prev_cpu)
+ new_cpu = wake_affine(tmp, p, cpu, prev_cpu, sync);
+
sd = NULL; /* Prefer wake_affine over balance flags */
- affine_sd = tmp;
break;
}

@@ -6657,16 +6659,11 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int sd_flag, int wake_f
sync_entity_load_avg(&p->se);

new_cpu = find_idlest_cpu(sd, p, cpu, prev_cpu, sd_flag);
- } else {
- if (affine_sd && cpu != prev_cpu)
- new_cpu = wake_affine(affine_sd, p, cpu, prev_cpu, sync);
+ } else if (sd_flag & SD_BALANCE_WAKE) { /* XXX always ? */
+ new_cpu = select_idle_sibling(p, prev_cpu, new_cpu);

- if (sd_flag & SD_BALANCE_WAKE) { /* XXX always ? */
- new_cpu = select_idle_sibling(p, prev_cpu, new_cpu);
-
- if (want_affine)
- current->recent_used_cpu = cpu;
- }
+ if (want_affine)
+ current->recent_used_cpu = cpu;
}
rcu_read_unlock();


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