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    SubjectRe: [PATCH] sched: fix lose fair sleeper bonus in switch_to_fair()
    On Tue, Sep 08, 2015 at 03:14:26PM +0900, Byungchul Park wrote:
    > On Tue, Sep 08, 2015 at 01:38:08PM +0800, Wanpeng Li wrote:
    > > On 9/8/15 1:28 PM, Byungchul Park wrote:
    > > >On Tue, Sep 08, 2015 at 11:46:01AM +0800, Wanpeng Li wrote:
    > > >>On 9/7/15 10:02 PM, Peter Zijlstra wrote:
    > > >>>Please always Cc at least the person who wrote the lines you modify.
    > > >>>
    > > >>>On Mon, Sep 07, 2015 at 05:45:20PM +0800, Wanpeng Li wrote:
    > > >>>>The sleeper task will be normalized when moved from fair_sched_class, in
    > > >>>>order that vruntime will be adjusted either the task is running or sleeping
    > > >>>>when moved back. The nomalization in switch_to_fair for sleep task will
    > > >>>>result in lose fair sleeper bonus in place_entity() once the vruntime -
    > > >>>>cfs_rq->min_vruntime is big when moved from fair_sched_class.
    > > >>>>
    > > >>>>This patch fix it by adjusting vruntime just during migrating as original
    > > >>>>codes since the vruntime of the task has usually NOT been normalized in
    > > >>>>this case.
    > > >>>Sorry, I cannot follow that at all. Maybe its me being sleep deprived,
    > > >>>but could you try that again?
    > > >>When changing away from the fair class while sleeping, relative
    > > >>vruntime is calculated to handle the case sleep when moved from
    > > >>fair_sched_class and running when moved to fair_sched_class. The
    > > >i don't think relative vruntime is calculated to handle the special case
    > > >you mentioned. i think the calculation is necessary for all cases detaching
    > >
    > > Please refer why the relative vruntime caculation is introduced to
    > > switched_from_fair(): https://lkml.org/lkml/2011/1/17/129
    >
    > hello,
    >
    > it is just a bug caused by not calculating a relative vruntime when
    > detached a task from cfs_rq, which is necessary though.
    >
    > >
    > > >a task from a cfs_rq.
    > > >
    > > >>absolute vruntime will be calculated in enqueue_entity() either the
    > > >>task is running or sleeping when moved back. The fair sleeper bonus
    > > >i think absolute vruntime is calculated in enqueue_entuty() only when the
    > > >task is on rq. therefore in the case that the task is not on rq,
    > > >switched_to_fair() has to calculate the absolute vruntime instread.
    > >
    > > Absolute vruntime is caculated in place_entity() which is called by
    > > enqueue_entity() for DEQUEUE_SLEEP task.
    >
    > as you may know, place_entity() is not for calculating an absolute
    > vruntime though.. anyway the important thing here is that, when a
    > sleeping task is moved back to fair class, enqueue_entity() for
    > DEQUEUE_SLEEP task won't be called.

    you may talk about calling enqueue_entity() when the task is woken up,
    not just when it is moved back. right?

    even if yes, i think place_entity() should not be used directly for
    calculating an absolute vruntime. it should be called after non/normalizing
    operations.

    >
    > thanks,
    > byungchul
    >
    > >
    > > Regards,
    > > Wanpeng Li
    > >
    > > >
    > > >>should be gained in place_entity() if the task is still sleeping.
    > > >>However, after recent commit ( 23ec30ddd7c1306: 'sched: add two
    > > >>functions for att(det)aching a task to(from) a cfs_rq'), the
    > > >>absolute vruntime will be calculated in switched_to_fair(), so the
    > > >>max_vruntime() which is called in place_entity() will select the
    > > >>absolute vruntime which is calculated in switched_to_fair() as the
    > > >>se->vruntime and lose the fair sleeper bonus.
    > > >please refer my another reply, and let me know if i missed something.
    > > >
    > > >thanks,
    > > >byungchul
    > > >
    > > >>Regards,
    > > >>Wanpeng Li
    > > >>
    > > >>>>Signed-off-by: Wanpeng Li <wanpeng.li@hotmail.com>
    > > >>>>---
    > > >>>> kernel/sched/fair.c | 11 +++++++----
    > > >>>> 1 files changed, 7 insertions(+), 4 deletions(-)
    > > >>>>
    > > >>>>diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
    > > >>>>index d26d3b7..eb9aa35 100644
    > > >>>>--- a/kernel/sched/fair.c
    > > >>>>+++ b/kernel/sched/fair.c
    > > >>>>@@ -8005,9 +8005,6 @@ static void attach_task_cfs_rq(struct task_struct *p)
    > > >>>> /* Synchronize task with its cfs_rq */
    > > >>>> attach_entity_load_avg(cfs_rq, se);
    > > >>>>-
    > > >>>>- if (!vruntime_normalized(p))
    > > >>>>- se->vruntime += cfs_rq->min_vruntime;
    > > >>>> }
    > > >>>> static void switched_from_fair(struct rq *rq, struct task_struct *p)
    > > >>>>@@ -8066,14 +8063,20 @@ void init_cfs_rq(struct cfs_rq *cfs_rq)
    > > >>>> #ifdef CONFIG_FAIR_GROUP_SCHED
    > > >>>> static void task_move_group_fair(struct task_struct *p)
    > > >>>> {
    > > >>>>+ struct sched_entity *se = &p->se;
    > > >>>>+ struct cfs_rq *cfs_rq = cfs_rq_of(se);
    > > >>>>+
    > > >>>> detach_task_cfs_rq(p);
    > > >>>> set_task_rq(p, task_cpu(p));
    > > >>>> #ifdef CONFIG_SMP
    > > >>>> /* Tell se's cfs_rq has been changed -- migrated */
    > > >>>>- p->se.avg.last_update_time = 0;
    > > >>>>+ se->avg.last_update_time = 0;
    > > >>>> #endif
    > > >>>> attach_task_cfs_rq(p);
    > > >>>>+
    > > >>>>+ if (!vruntime_normalized(p))
    > > >>>>+ se->vruntime += cfs_rq->min_vruntime;
    > > >>>> }
    > > >>>> void free_fair_sched_group(struct task_group *tg)
    > > >>>>--
    > > >>>>1.7.1
    > > >>>>
    > > >>--
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