Re: [PATCH] sched: pull tasks when CPU is about to run SCHED_IDLE tasks

From: Vincent Guittot
Date: Thu Feb 04 2021 - 03:05:00 EST

On Thu, 4 Feb 2021 at 04:57, Jiang Biao <benbjiang@xxxxxxxxx> wrote:
>
> Hi,
>
> On Wed, 3 Feb 2021 at 19:17, chin <ultrachin@xxxxxxx> wrote:
> >
> >
> >
> >
> > At 2021-02-02 23:54:15, "Vincent Guittot" <vincent.guittot@xxxxxxxxxx> wrote:
> > >On Tue, 2 Feb 2021 at 08:56, chin <ultrachin@xxxxxxx> wrote:
> > >>
> > >>
> > >>
> > >>
> > >> At 2021-01-13 16:30:14, "Vincent Guittot" <vincent.guittot@xxxxxxxxxx> wrote:
> > >> >On Wed, 13 Jan 2021 at 04:14, chin <ultrachin@xxxxxxx> wrote:
> > >> >>
> > >> >>
> > >> >>
> > >> >>
> > >> >> At 2021-01-12 16:18:51, "Vincent Guittot" <vincent.guittot@xxxxxxxxxx> wrote:
> > >> >> >On Tue, 12 Jan 2021 at 07:59, chin <ultrachin@xxxxxxx> wrote:
> > >> >> >>
> > >> >> >>
> > >> >> >>
> > >> >> >>
> > >> >> >> At 2021-01-11 19:04:19, "Vincent Guittot" <vincent.guittot@xxxxxxxxxx> wrote:
> > >> >> >> >On Mon, 11 Jan 2021 at 09:27, chin <ultrachin@xxxxxxx> wrote:
> > >> >> >> >>
> > >> >> >> >>
> > >> >> >> >> At 2020-12-23 19:30:26, "Vincent Guittot" <vincent.guittot@xxxxxxxxxx> wrote:
> > >> >> >> >> >On Wed, 23 Dec 2020 at 09:32, <ultrachin@xxxxxxx> wrote:
> > >> >> >> >> >>
> > >> >> >> >> >> From: Chen Xiaoguang <xiaoggchen@xxxxxxxxxxx>
> > >> >> >> >> >>
> > >> >> >> >> >> 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@xxxxxxxxxxx>
> > >> >> >> >> >> Signed-off-by: Chen He <heddchen@xxxxxxxxxxx>
> > >> >> >> >> >> ---
> > >> >> >> >> >> 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