On Tue, 2 Feb 2021 at 08:56, chin <ultrac...@163.com> wrote: > > > > > At 2021-01-13 16:30:14, "Vincent Guittot" <vincent.guit...@linaro.org> wrote: > >On Wed, 13 Jan 2021 at 04:14, chin <ultrac...@163.com> wrote: > >> > >> > >> > >> > >> At 2021-01-12 16:18:51, "Vincent Guittot" <vincent.guit...@linaro.org> > >> wrote: > >> >On Tue, 12 Jan 2021 at 07:59, chin <ultrac...@163.com> wrote: > >> >> > >> >> > >> >> > >> >> > >> >> At 2021-01-11 19:04:19, "Vincent Guittot" <vincent.guit...@linaro.org> > >> >> wrote: > >> >> >On Mon, 11 Jan 2021 at 09:27, chin <ultrac...@163.com> wrote: > >> >> >> > >> >> >> > >> >> >> At 2020-12-23 19:30:26, "Vincent Guittot" > >> >> >> <vincent.guit...@linaro.org> wrote: > >> >> >> >On Wed, 23 Dec 2020 at 09:32, <ultrac...@163.com> wrote: > >> >> >> >> > >> >> >> >> From: Chen Xiaoguang <xiaoggc...@tencent.com> > >> >> >> >> > >> >> >> >> 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 <xiaoggc...@tencent.com> > >> >> >> >> Signed-off-by: Chen He <heddc...@tencent.com> > >> >> >> >> --- > >> >> >> >> 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 > > /* > * 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 > > > >> By doing this SCHED_NORMAL tasks waiting on other cpus would get > >> a chance to be pulled to this cpu and run, it is helpful to reduce the > >> latency > >> of SCHED_NORMAL tasks. > >> > >> > >> >>> > >> >> > > >> >> >> > >> >> >> > > >> >> >> > >> >> >> >Also why not doing that for default case too ? i.e. balance_fair() ? > >> >> >> You are right, if you think this scenario makes sense, we will send a > >> >> >> refined patch soon :-) > >> >> >> > >> >> >> > > >> >> >> >> + sched_idle_cpu(rq->cpu)) > >> >> >> >> + goto idle; > >> >> >> >> + > >> >> >> >> again: > >> >> >> >> if (!sched_fair_runnable(rq)) > >> >> >> >> goto idle; > >> >> >> >> -- > >> >> >> >> 1.8.3.1 > >> >> >> >> > >> >> >> >>
