Messages in this thread Patch in this message | | | Date | Thu, 14 Jul 2022 21:43:13 +0800 | Subject | Re: [External] Re: [PATCH v2 09/10] sched/fair: stop load tracking when task switched_from_fair() | From | Chengming Zhou <> |
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On 2022/7/14 20:33, Dietmar Eggemann wrote: > On 13/07/2022 06:04, Chengming Zhou wrote: >> The same reason as the previous commit, if we don't reset the >> sched_avg last_update_time to 0, after a while in switched_to_fair(): >> >> switched_to_fair >> attach_task_cfs_rq >> attach_entity_cfs_rq >> update_load_avg >> __update_load_avg_se(now, cfs_rq, se) >> >> The delta (now - sa->last_update_time) will wrongly contribute/decay >> sched_avg depends on the task running/runnable status at that time. > > IMHO, a queued !fair task when switching back to fair will already be > enqueued (attached) as a fair task in __sched_setscheduler() prior to > the check_class_changed() call.
Right, this is true for a queued !fair task, it will enqueued (attached) before check_class_changed().
enqueue_task_fair() enqueue_entity() update_load_avg(cfs_rq, se, UPDATE_TG | DO_ATTACH) if (se->avg.last_update_time && !(flags & SKIP_AGE_LOAD)) --> true __update_load_avg_se(now, cfs_rq, se) --> (1) check_class_changed() switched_to_fair() attach_task_cfs_rq() attach_entity_cfs_rq() update_load_avg(cfs_rq, se, sched_feat(ATTACH_AGE_LOAD) ? 0 : SKIP_AGE_LOAD) if (se->avg.last_update_time && !(flags & SKIP_AGE_LOAD)) --> true __update_load_avg_se(now, cfs_rq, se) --> (2)
1. for queued !fair: (1) delta = (now - last_update_time), last_update_time is the time when switched_from_fair().
2. for !queued !fair: (2) delta = (now - last_update_time), last_update_time is the time when switched_from_fair().
The scenario in the commit message only cover !queued !fair case, I forget the queued !fair case, their problem is the same.
> > I can't see how this will work with your proposed change in using > last_update_time=0 for fair->!fair->fair class changes? If we reset last_update_time=0 for !fair task, then:
1. for queued !fair: will not do (1) since the if condition is false.
2. for !queued !fair: will not do (2) since the if condition is false.
Thanks.
> >> This patch reset it's sched_avg last_update_time to 0, stop load >> tracking for !fair task, later in switched_to_fair() -> >> update_load_avg(), we can use its saved sched_avg. >> >> Signed-off-by: Chengming Zhou <zhouchengming@bytedance.com> >> --- >> kernel/sched/fair.c | 5 +++++ >> 1 file changed, 5 insertions(+) >> >> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c >> index 50f65a2ede32..576028f5a09e 100644 >> --- a/kernel/sched/fair.c >> +++ b/kernel/sched/fair.c >> @@ -11552,6 +11552,11 @@ static void attach_task_cfs_rq(struct task_struct *p) >> static void switched_from_fair(struct rq *rq, struct task_struct *p) >> { >> detach_task_cfs_rq(p); >> + >> +#ifdef CONFIG_SMP >> + /* Stop load tracking for !fair task */ >> + p->se.avg.last_update_time = 0; >> +#endif >> } >> >> static void switched_to_fair(struct rq *rq, struct task_struct *p) >
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