On Thu, 4 Feb 2021 at 04:57, Jiang Biao <[email protected]> wrote: > > Hi, > > On Wed, 3 Feb 2021 at 19:17, chin <[email protected]> wrote: > > > > > > > > > > At 2021-02-02 23:54:15, "Vincent Guittot" <[email protected]> > > wrote: > > >On Tue, 2 Feb 2021 at 08:56, chin <[email protected]> wrote: > > >> > > >> > > >> > > >> > > >> At 2021-01-13 16:30:14, "Vincent Guittot" <[email protected]> > > >> wrote: > > >> >On Wed, 13 Jan 2021 at 04:14, chin <[email protected]> wrote: > > >> >> > > >> >> > > >> >> > > >> >> > > >> >> At 2021-01-12 16:18:51, "Vincent Guittot" > > >> >> <[email protected]> wrote: > > >> >> >On Tue, 12 Jan 2021 at 07:59, chin <[email protected]> wrote: > > >> >> >> > > >> >> >> > > >> >> >> > > >> >> >> > > >> >> >> At 2021-01-11 19:04:19, "Vincent Guittot" > > >> >> >> <[email protected]> wrote: > > >> >> >> >On Mon, 11 Jan 2021 at 09:27, chin <[email protected]> wrote: > > >> >> >> >> > > >> >> >> >> > > >> >> >> >> At 2020-12-23 19:30:26, "Vincent Guittot" > > >> >> >> >> <[email protected]> wrote: > > >> >> >> >> >On Wed, 23 Dec 2020 at 09:32, <[email protected]> wrote: > > >> >> >> >> >> > > >> >> >> >> >> From: Chen Xiaoguang <[email protected]> > > >> >> >> >> >> > > >> >> >> >> >> Before a CPU switches from running SCHED_NORMAL task to > > >> >> >> >> >> SCHED_IDLE task, trying to pull SCHED_NORMAL tasks from other > > >> >> >> >> > > > >> >> >> >> >Could you explain more in detail why you only care about this > > >> >> >> >> >use case > > >> >> >> >> > > >> >> >> >> >in particular and not the general case? > > >> >> >> >> > > >> >> >> >> > > >> >> >> >> We want to run online tasks using SCHED_NORMAL policy and > > >> >> >> >> offline tasks > > >> >> >> >> using SCHED_IDLE policy. The online tasks and the offline tasks > > >> >> >> >> run in > > >> >> >> >> the same computer in order to use the computer efficiently. > > >> >> >> >> The online tasks are in sleep in most times but should responce > > >> >> >> >> soon once > > >> >> >> >> wake up. The offline tasks are in low priority and will run > > >> >> >> >> only when no online > > >> >> >> >> tasks. > > >> >> >> >> > > >> >> >> >> The online tasks are more important than the offline tasks and > > >> >> >> >> are latency > > >> >> >> >> sensitive we should make sure the online tasks preempt the > > >> >> >> >> offline tasks > > >> >> >> >> as soon as possilbe while there are online tasks waiting to run. > > >> >> >> >> So in our situation we hope the SCHED_NORMAL to run if has any. > > >> >> >> >> > > >> >> >> >> Let's assume we have 2 CPUs, > > >> >> >> >> In CPU1 we got 2 SCHED_NORMAL tasks. > > >> >> >> >> in CPU2 we got 1 SCHED_NORMAL task and 2 SCHED_IDLE tasks. > > >> >> >> >> > > >> >> >> >> CPU1 CPU2 > > >> >> >> >> curr rq1 curr rq2 > > >> >> >> >> +------+ | +------+ +------+ | +----+ +----+ > > >> >> >> >> t0 |NORMAL| | |NORMAL| |NORMAL| | |IDLE| |IDLE| > > >> >> >> >> +------+ | +------+ +------+ | +----+ +----+ > > >> >> >> >> > > >> >> >> >> NORMAL exits or blocked > > >> >> >> >> +------+ | +------+ | +----+ +----+ > > >> >> >> >> t1 |NORMAL| | |NORMAL| | |IDLE| |IDLE| > > >> >> >> >> +------+ | +------+ | +----+ +----+ > > >> >> >> >> > > >> >> >> >> pick_next_task_fair > > >> >> >> >> +------+ | +------+ +----+ | +----+ > > >> >> >> >> t2 |NORMAL| | |NORMAL| |IDLE| | |IDLE| > > >> >> >> >> +------+ | +------+ +----+ | +----+ > > >> >> >> >> > > >> >> >> >> SCHED_IDLE running > > >> >> >> >> t3 +------+ | +------+ +----+ | +----+ > > >> >> >> >> |NORMAL| | |NORMAL| |IDLE| | |IDLE| > > >> >> >> >> +------+ | +------+ +----+ | +----+ > > >> >> >> >> > > >> >> >> >> run_rebalance_domains > > >> >> >> >> +------+ | +------+ | +----+ +----+ > > >> >> >> >> t4 |NORMAL| | |NORMAL| | |IDLE| |IDLE| > > >> >> >> >> +------+ | +------+ | +----+ +----+ > > >> >> >> >> > > >> >> >> >> As we can see > > >> >> >> >> t1: NORMAL task in CPU2 exits or blocked > > >> >> >> >> t2: CPU2 pick_next_task_fair would pick a SCHED_IDLE to run > > >> >> >> >> while > > >> >> >> >> another SCHED_NORMAL in rq1 is waiting. > > >> >> >> >> t3: SCHED_IDLE run in CPU2 while a SCHED_NORMAL wait in CPU1. > > >> >> >> >> t4: after a short time, periodic load_balance triggerd and pull > > >> >> >> >> SCHED_NORMAL in rq1 to rq2, and SCHED_NORMAL likely preempts > > >> >> >> >> SCHED_IDLE. > > >> >> >> >> > > >> >> >> >> In this scenario, SCHED_IDLE is running while SCHED_NORMAL is > > >> >> >> >> waiting to run. > > >> >> >> >> The latency of this SCHED_NORMAL will be high which is not > > >> >> >> >> acceptble. > > >> >> >> >> > > >> >> >> >> Do a load_balance before running the SCHED_IDLE may fix this > > >> >> >> >> problem. > > >> >> >> >> > > >> >> >> >> This patch works as below: > > >> >> >> >> > > >> >> >> >> CPU1 CPU2 > > >> >> >> >> curr rq1 curr rq2 > > >> >> >> >> +------+ | +------+ +------+ | +----+ +----+ > > >> >> >> >> t0 |NORMAL| | |NORMAL| |NORMAL| | |IDLE| |IDLE| > > >> >> >> >> +------+ | +------+ +------+ | +----+ +----+ > > >> >> >> >> > > >> >> >> >> NORMAL exits or blocked > > >> >> >> >> +------+ | +------+ | +----+ +----+ > > >> >> >> >> t1 |NORMAL| | |NORMAL| | |IDLE| |IDLE| > > >> >> >> >> +------+ | +------+ | +----+ +----+ > > >> >> >> >> > > >> >> >> >> t2 pick_next_task_fair (all se are > > >> >> >> >> SCHED_IDLE) > > >> >> >> >> > > >> >> >> >> newidle_balance > > >> >> >> >> +------+ | +------+ | +----+ +----+ > > >> >> >> >> t3 |NORMAL| | |NORMAL| | |IDLE| |IDLE| > > >> >> >> >> +------+ | +------+ | +----+ +----+ > > >> >> >> >> > > >> >> >> >> > > >> >> >> >> t1: NORMAL task in CPU2 exits or blocked > > >> >> >> >> t2: pick_next_task_fair check all se in rbtree are SCHED_IDLE > > >> >> >> >> and calls > > >> >> >> >> newidle_balance who tries to pull a SCHED_NORMAL(if has). > > >> >> >> >> t3: pick_next_task_fair would pick a SCHED_NORMAL to run > > >> >> >> >> instead of > > >> >> >> >> SCHED_IDLE(likely). > > >> >> >> >> > > >> >> >> >> > > > >> >> >> >> >> CPU by doing load_balance first. > > >> >> >> >> >> > > >> >> >> >> >> Signed-off-by: Chen Xiaoguang <[email protected]> > > >> >> >> >> >> Signed-off-by: Chen He <[email protected]> > > >> >> >> >> >> --- > > >> >> >> >> >> kernel/sched/fair.c | 5 +++++ > > >> >> >> >> >> 1 file changed, 5 insertions(+) > > >> >> >> >> >> > > >> >> >> >> >> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > > >> >> >> >> >> index ae7ceba..0a26132 100644 > > >> >> >> >> >> --- a/kernel/sched/fair.c > > >> >> >> >> >> +++ b/kernel/sched/fair.c > > >> >> >> >> >> @@ -7004,6 +7004,11 @@ struct task_struct * > > >> >> >> >> >> struct task_struct *p; > > >> >> >> >> >> int new_tasks; > > >> >> >> >> >> > > >> >> >> >> >> + if (prev && > > >> >> >> >> >> + fair_policy(prev->policy) && > > >> >> >> >> > > > >> >> >> >> >Why do you need a prev and fair task ? You seem to target the > > >> >> >> >> >special > > >> >> >> >> >case of pick_next_task but in this case why not only testing > > >> >> >> >> >rf!=null > > >> >> >> >> > to make sure to not return immediately after jumping to the > > >> >> >> >> > idle > > >> >> >> >> > > >> >> >> >> >label? > > >> >> >> >> We just want to do load_balance only when CPU switches from > > >> >> >> >> SCHED_NORMAL > > >> >> >> >> to SCHED_IDLE. > > >> >> >> >> If not check prev, when the running tasks are all SCHED_IDLE, > > >> >> >> >> we would > > >> >> >> >> do newidle_balance everytime in pick_next_task_fair, it makes > > >> >> >> >> no sense > > >> >> >> >> and kind of wasting. > > >> >> >> > > > >> >> >> >I agree that calling newidle_balance every time > > >> >> >> >pick_next_task_fair is > > >> >> >> >called when there are only sched_idle tasks is useless. > > >> >> >> >But you also have to take into account cases where there was > > >> >> >> >another > > >> >> >> >class of task running on the cpu like RT one. In your example > > >> >> >> >above, > > >> >> >> >if you replace the normal task on CPU2 by a RT task, you still > > >> >> >> >want to > > >> >> >> > > >> >> >> >pick the normal task on CPU1 once RT task goes to sleep. > > >> >> >> Sure, this case should be taken into account, we should also try > > >> >> >> to > > >> >> >> pick normal task in this case. > > >> >> >> > > >> >> >> > > > >> >> >> >Another point that you will have to consider the impact on > > >> >> >> >rq->idle_stamp because newidle_balance is assumed to be called > > >> >> >> >before > > >> >> >> > > >> >> >> >going idle which is not the case anymore with your use case > > >> >> >> Yes. rq->idle_stamp should not be changed in this case. > > >> >> >> > > >> >> >> > > >> >> >> > > >> >> >> Actually we want to pull a SCHED_NORMAL task (if possible) to run > > >> >> >> when a cpu is > > >> >> >> about to run SCHED_IDLE task. But currently newidle_balance is not > > >> >> >> designed for SCHED_IDLE so SCHED_IDLE can also be pulled which > > >> >> >> is useless in our situation. > > >> >> > > > >> >> >newidle_balance will pull a sched_idle task only if there is an > > >> >> >imbalance which is the right thing to do IMO to ensure fairness > > >> >> >between sched_idle tasks. Being a sched_idle task doesn't mean that > > >> >> >we should break the fairness > > >> >> > > > >> >> >> > > >> >> >> So we plan to add a new function sched_idle_balance which only try > > >> >> >> to > > >> >> >> pull SCHED_NORMAL tasks from the busiest cpu. And we will call > > >> >> >> sched_idle_balance when the previous task is normal or RT and > > >> >> >> hoping we can pull a SCHED_NORMAL task to run. > > >> >> >> > > >> >> >> Do you think it is ok to add a new sched_idle_balance? > > >> >> > > > >> >> >I don't see any reason why the scheduler should not pull a sched_idle > > >> >> >task if there is an imbalance. That will happen anyway during the > > >> >> >next > > >> >> > > >> >> >periodic load balance > > >> >> OK. We should not pull the SCHED_IDLE tasks only in load_balance. > > >> >> > > >> >> > > >> >> Do you think it make sense to do an extra load_balance when cpu is > > >> >> about to run SCHED_IDLE task (switched from normal/RT)? > > >> > > > >> >I'm not sure to get your point here. > > >> >Do you mean if a sched_idle task is picked to become the running task > > >> >whereas there are runnable normal tasks ? This can happen if normal > > >> >tasks are long running tasks. We should not in this case. The only > > >> >case is when the running task, which is not a sched_idle task but a > > >> >normal/rt/deadline one, goes to sleep and there are only sched_idle > > >> >tasks enqueued. In this case and only in this case, we should trigger > > >> >a load_balance to get a chance to pull a waiting normal task from > > >> >another CPU. > > >> > > > >> >This means checking this state in pick_next_task_fair() and in > > >> >balance_fair() > > >> > > >> We made another change would you please give some comments? > > >> > > >> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > > >> index 04a3ce2..2357301 100644 > > >> --- a/kernel/sched/fair.c > > >> +++ b/kernel/sched/fair.c > > >> @@ -7029,6 +7029,10 @@ struct task_struct * > > >> struct task_struct *p; > > >> int new_tasks; > > >> > > >> + if (sched_idle_rq(rq) && prev && prev->state && > > >> + prev->policy != SCHED_IDLE) > > >> + goto idle; > > >> + > > >> again: > > >> if (!sched_fair_runnable(rq)) > > >> goto idle; > > >> @@ -10571,7 +10575,8 @@ static int newidle_balance(struct rq *this_rq, > > >> struct rq_flags *rf) > > >> * We must set idle_stamp _before_ calling idle_balance(), such > > >> that we > > >> * measure the duration of idle_balance() as idle time. > > >> */ > > >> - this_rq->idle_stamp = rq_clock(this_rq); > > >> + if (!rq->nr_running) > > >> + this_rq->idle_stamp = rq_clock(this_rq); > > > > > >I know that I asked you to take care of not setting idle_stamp during > > >the last review. But I forgot that it was cleared anyway at the end of > > >newidle_balance() if there is some tasks running on the cpu so this is > > > > >not needed and make the code less readable > > Yes, the idle_stamp was cleared. > > > > > > > >> > > >> /* > > >> * Do not pull tasks towards !active CPUs... > > >> > > > > > >I don't see the change for balance_fair() > > >When a rt task goes back to sleep and there is only sched_idle tasks > > > > >as an example > > > > > > Yes, we should consider this situation too. > > How about this one ? > > > > > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > > index 04a3ce2..982b842 100644 > > --- a/kernel/sched/fair.c > > +++ b/kernel/sched/fair.c > > @@ -6849,6 +6849,9 @@ static void task_dead_fair(struct task_struct *p) > > static int > > balance_fair(struct rq *rq, struct task_struct *prev, struct rq_flags *rf) > > { > > + if (sched_idle_rq(rq)) > > + return newidle_balance(rq, rf) != 0; > > + > > if (rq->nr_running) > > return 1; > > > Maybe we'd better merge the branches? like, > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > index ac950ac950bc..259deda79c06 100644 > --- a/kernel/sched/fair.c > +++ b/kernel/sched/fair.c > @@ -6852,7 +6852,7 @@ static void task_dead_fair(struct task_struct *p) > static int > balance_fair(struct rq *rq, struct task_struct *prev, struct rq_flags *rf) > { > - if (rq->nr_running) > + if (rq->nr_running && !sched_idle_rq(rq))
yes this looks better > return 1; > > return newidle_balance(rq, rf) != 0; > > Thx. > Regards, > Jiang
