Messages in this thread | | | From | Vincent Guittot <> | Date | Tue, 12 Jan 2021 09:18:51 +0100 | Subject | Re: [PATCH] sched: pull tasks when CPU is about to run SCHED_IDLE tasks |
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On Tue, 12 Jan 2021 at 07:59, chin <ultrachin@163.com> wrote: > > > > > At 2021-01-11 19:04:19, "Vincent Guittot" <vincent.guittot@linaro.org> wrote: > >On Mon, 11 Jan 2021 at 09:27, chin <ultrachin@163.com> wrote: > >> > >> > >> At 2020-12-23 19:30:26, "Vincent Guittot" <vincent.guittot@linaro.org> wrote: > >> >On Wed, 23 Dec 2020 at 09:32, <ultrachin@163.com> wrote: > >> >> > >> >> From: Chen Xiaoguang <xiaoggchen@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 <xiaoggchen@tencent.com> > >> >> Signed-off-by: Chen He <heddchen@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
> > > > >> > >> > > >> > >> >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 > >> >> > >> >>
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