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SubjectRe: [PATCH] sched: fix unfairness when upgrade weight
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On Wed, 2009-02-25 at 15:32 +0800, Miao Xie wrote:

> This patch fixes this bug by tuning the vruntime of weight-upgraded
> sched entities, just like waking up a task. the new vruntime will be
> cfs_rq->min_vruntime + sched_vslice();

I really don't like that.

Better would be to scale with min_vruntime, which would at least
approximate the lag somewhat.

Best is to compute the actual lag, but that might just not be worth the
extra overhead.

http://programming.kicks-ass.net/kernel-patches/sched-avg_vruntime/


> ---
> kernel/sched.c | 16 +++++++++-------
> kernel/sched_fair.c | 9 +++++++++
> 2 files changed, 18 insertions(+), 7 deletions(-)
>
> diff --git a/kernel/sched.c b/kernel/sched.c
> index 410eec4..26e6d33 100644
> --- a/kernel/sched.c
> +++ b/kernel/sched.c
> @@ -5096,12 +5096,8 @@ void set_user_nice(struct task_struct *p, long nice)
>
> if (on_rq) {
> enqueue_task(rq, p, 0);
> - /*
> - * If the task increased its priority or is running and
> - * lowered its priority, then reschedule its CPU:
> - */
> - if (delta < 0 || (delta > 0 && task_running(rq, p)))
> - resched_task(rq->curr);
> + p->sched_class->prio_changed(rq, p, old_prio,
> + task_running(rq, p));
> }
> out_unlock:
> task_rq_unlock(rq, &flags);
> @@ -8929,16 +8925,22 @@ static void __set_se_shares(struct sched_entity *se, unsigned long shares)
> {
> struct cfs_rq *cfs_rq = se->cfs_rq;
> int on_rq;
> + unsigned long old_weight;
>
> on_rq = se->on_rq;
> if (on_rq)
> dequeue_entity(cfs_rq, se, 0);
>
> + old_weight = se->load.weight;
> se->load.weight = shares;
> se->load.inv_weight = 0;
>
> - if (on_rq)
> + if (on_rq) {
> + if (se->load.weight > old_weight)
> + se->vruntime = cfs_rq->min_vruntime
> + + sched_vslice(cfs_rq, se);
> enqueue_entity(cfs_rq, se, 0);
> + }
> }
>
> static void set_se_shares(struct sched_entity *se, unsigned long shares)
> diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c
> index 0566f2a..34d4d11 100644
> --- a/kernel/sched_fair.c
> +++ b/kernel/sched_fair.c
> @@ -1690,6 +1690,15 @@ static void task_new_fair(struct rq *rq, struct task_struct *p)
> static void prio_changed_fair(struct rq *rq, struct task_struct *p,
> int oldprio, int running)
> {
> + struct cfs_rq *cfs_rq = task_cfs_rq(p);
> + struct sched_entity *se = &p->se;
> + int on_rq = se->on_rq;
> +
> + if (p->prio < oldprio && on_rq) {
> + dequeue_entity(cfs_rq, se, 0);
> + se->vruntime = cfs_rq->min_vruntime + sched_vslice(cfs_rq, se);
> + enqueue_entity(cfs_rq, se, 0);
> + }

we very likely just enqueued the thing, and now we dequeue/enqueue
again.. not very nice.

> /*
> * Reschedule if we are currently running on this runqueue and
> * our priority decreased, or if we are not currently running on



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