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SubjectRe: [PATCH] sched: Accelerate "pick_next_entity" under special condition
On 01/16/2012 05:37 PM, Michael Wang wrote:

> From: Michael Wang <wangyun@linux.vnet.ibm.com>
>
> We can avoid some useless operation in some special condition.
>
> For example:
> If we have "cfs_rq->next" and it can be use, we just return it directly.
>


Please tell me if I got wrong understanding on these code, I think the change can
maintain the old logic and suppose to be a little quick in some condition.


> Signed-off-by: Michael Wang <wangyun@linux.vnet.ibm.com>
> ---
> kernel/sched/fair.c | 28 +++++++++++++++++-----------
> 1 files changed, 17 insertions(+), 11 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 84adb2d..9fc2c3c 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -1295,6 +1295,8 @@ set_next_entity(struct cfs_rq *cfs_rq, struct sched_entity *se)
> static int
> wakeup_preempt_entity(struct sched_entity *curr, struct sched_entity *se);
>
> +#define ENTITY_PREEMPT_ALLOWED(prev,next) (wakeup_preempt_entity(prev, next) < 1)


I want to use a name which can make it more easy to be understand, please tell me
if it is a bad idea...

Regards,
Michael Wang

> +
> /*
> * Pick the next process, keeping these things in mind, in this order:
> * 1) keep things fair between processes/task groups
> @@ -1308,29 +1310,33 @@ static struct sched_entity *pick_next_entity(struct cfs_rq *cfs_rq)
> struct sched_entity *left = se;
>
> /*
> - * Avoid running the skip buddy, if running something else can
> - * be done without getting too unfair.
> + * Someone really wants this to run. If it's not unfair, run it.
> */
> - if (cfs_rq->skip == se) {
> - struct sched_entity *second = __pick_next_entity(se);
> - if (second && wakeup_preempt_entity(second, left) < 1)
> - se = second;
> + if (cfs_rq->next && ENTITY_PREEMPT_ALLOWED(cfs_rq->next, left)) {
> + se = cfs_rq->next;
> + goto out;
> }
>
> /*
> * Prefer last buddy, try to return the CPU to a preempted task.
> */
> - if (cfs_rq->last && wakeup_preempt_entity(cfs_rq->last, left) < 1)
> + if (cfs_rq->last && ENTITY_PREEMPT_ALLOWED(cfs_rq->last, left)) {
> se = cfs_rq->last;
> + goto out;
> + }
>
> /*
> - * Someone really wants this to run. If it's not unfair, run it.
> + * Avoid running the skip buddy, if running something else can
> + * be done without getting too unfair.
> */
> - if (cfs_rq->next && wakeup_preempt_entity(cfs_rq->next, left) < 1)
> - se = cfs_rq->next;
> + if (cfs_rq->skip == se) {
> + struct sched_entity *second = __pick_next_entity(se);
> + if (second && ENTITY_PREEMPT_ALLOWED(second, left))
> + se = second;
> + }
>
> +out:
> clear_buddies(cfs_rq, se);
> -
> return se;
> }
>




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