Messages in this thread | | | From | Yafang Shao <> | Date | Thu, 3 Jun 2021 09:29:28 +0800 | Subject | Re: [PATCH 1/1] sched: do active load balance on the new idle cpu |
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On Wed, Jun 2, 2021 at 9:26 PM Vincent Guittot <vincent.guittot@linaro.org> wrote: > > On Wed, 2 Jun 2021 at 14:58, Yafang Shao <laoar.shao@gmail.com> wrote: > > > > On Wed, Jun 2, 2021 at 8:37 PM Vincent Guittot > > <vincent.guittot@linaro.org> wrote: > > > > > > On Wed, 2 Jun 2021 at 14:26, Yafang Shao <laoar.shao@gmail.com> wrote: > > > > > > > > We monitored our latency-sensitive RT tasks are randomly preempted by the > > > > kthreads migration/n, which means to migrate tasks on CPUn to other new > > > > idle CPU. The logical as follows, > > > > > > > > new idle CPU CPU n > > > > (no task to run) (busy running) > > > > wakeup migration/n (busy running) > > > > (idle) migraion/n preempts current task > > > > run the migrated task (busy running) > > > > > > migration thread is only used when we want to migrate the currently > > > running task of the source cpu. > > > > Could you pls explain it in detail ? > > CPU A > become idle > call newidle_balance() > ... > load_balance() > ... CPU B is the busiest cpu > env.src_cpu = CPU B; > > if (busiest->nr_running > 1) { > ... > There is more than 1 runnable threads on CPU B > Try to migrate cfs runnable but not running tasks from CPU B to CPU A > in your case, the migration of cfs task should happen here > because the RT task is running > > Handle case of pinned tasks > > > if (!ld_moved) > no runnable but not running task was moved so we might want to > migrate the current running task > > need_active_balance() should not return true in your case > because tasks should have been migrated during the step above > > wake up stop/migration thread to preempt the current running > thread so we can migrate it >
Thanks for the explanation.
> > so you have has a UC which doesn't migrate task in the 1st step when > trying to pull runnable and not running tasks but it makes > need_active_balance() return true. Woudl be good to know which > condition makes need_active_balance() to return true >
I will analyze why need_active_balance() returns true.
> > > But I find the migration/n will pick a task from src_rq->cfs_tasks > > rather than the current running task, see also detach_one_task(): > > The current running task is migration/n one at that time > > > > > detach_one_task > > list_for_each_entry_reverse(p, &env->src_rq->cfs_tasks, se.group_node) { > > detach_task(p, env); > > } > > > > > > > This doesn't seem to be your case as it's a RT thread that is > > > currently running so the migration thread should not be woken up as we > > > don't need it to migrate a runnable but not running cfs thread from > > > coin to new idle CPU > > > > > > Do you have more details about the UC. Could it be a race between new > > > idle load balance starting migration thread to pull the cfs running > > > thread and the RT thread waking up and preempting cfs task before > > > migration threads which then preempt your RT threads > > > > > > > > > > No, it is not a race. Below is the detail from sched:sched_swith tracepoint: > > > > sensing_node-8880 [007] d... 4300.544185: sched_switch: > > prev_comm=sensing_node prev_pid=8880 prev_prio=98 prev_state=S ==> > > next_comm=sensing_node next_pid=8897 next_prio=98 > > sensing_node-8897 [007] d... 4300.544214: sched_switch: > > prev_comm=sensing_node prev_pid=8897 prev_prio=98 prev_state=S ==> > > next_comm=sensing_node next_pid=8880 next_prio=98 > > sensing_node-8880 [007] d... 4300.544506: sched_switch: > > prev_comm=sensing_node prev_pid=8880 prev_prio=98 prev_state=R ==> > > next_comm=migration/7 next_pid=47 next_prio=0 > > migration/7-47 [007] d... 4300.544509: sched_switch: > > prev_comm=migration/7 prev_pid=47 prev_prio=0 prev_state=S ==> > > next_comm=sensing_node next_pid=8880 next_prio=98 > > > > sensing_node is a RR task and it was preempted by migration/7. > > > > > > > > > > > > > As the new idle CPU is going to be idle, we'd better move the migration > > > > work on it instead of burdening the busy CPU. After this change, the > > > > logic is, > > > > new idle CPU CPU n > > > > (no task to run) (busy running) > > > > migrate task from CPU n (busy running) > > > > run the migrated task (busy running) > > > > > > > > Signed-off-by: Yafang Shao <laoar.shao@gmail.com> > > > > --- > > > > kernel/sched/fair.c | 17 +++++------------ > > > > 1 file changed, 5 insertions(+), 12 deletions(-) > > > > > > > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > > > > index 3248e24a90b0..3e8b98b982ff 100644 > > > > --- a/kernel/sched/fair.c > > > > +++ b/kernel/sched/fair.c > > > > @@ -9807,13 +9807,11 @@ static int load_balance(int this_cpu, struct rq *this_rq, > > > > busiest->push_cpu = this_cpu; > > > > active_balance = 1; > > > > } > > > > - raw_spin_unlock_irqrestore(&busiest->lock, flags); > > > > > > > > - if (active_balance) { > > > > - stop_one_cpu_nowait(cpu_of(busiest), > > > > - active_load_balance_cpu_stop, busiest, > > > > - &busiest->active_balance_work); > > > > - } > > > > + if (active_balance) > > > > + active_load_balance_cpu_stop(busiest); > > > > > > this doesn't make sense because we reach this point if we want to > > > migrate the current running task of the busiest cpu and in order to do > > > this we need the preempt this current running thread > > > > > > > + > > > > + raw_spin_unlock_irqrestore(&busiest->lock, flags); > > > > } > > > > } else { > > > > sd->nr_balance_failed = 0; > > > > @@ -9923,7 +9921,6 @@ static int active_load_balance_cpu_stop(void *data) > > > > struct task_struct *p = NULL; > > > > struct rq_flags rf; > > > > > > > > - rq_lock_irq(busiest_rq, &rf); > > > > /* > > > > * Between queueing the stop-work and running it is a hole in which > > > > * CPUs can become inactive. We should not move tasks from or to > > > > @@ -9933,8 +9930,7 @@ static int active_load_balance_cpu_stop(void *data) > > > > goto out_unlock; > > > > > > > > /* Make sure the requested CPU hasn't gone down in the meantime: */ > > > > - if (unlikely(busiest_cpu != smp_processor_id() || > > > > - !busiest_rq->active_balance)) > > > > + if (unlikely(!busiest_rq->active_balance)) > > > > goto out_unlock; > > > > > > > > /* Is there any task to move? */ > > > > @@ -9981,13 +9977,10 @@ static int active_load_balance_cpu_stop(void *data) > > > > rcu_read_unlock(); > > > > out_unlock: > > > > busiest_rq->active_balance = 0; > > > > - rq_unlock(busiest_rq, &rf); > > > > > > > > if (p) > > > > attach_one_task(target_rq, p); > > > > > > > > - local_irq_enable(); > > > > - > > > > return 0; > > > > } > > > > > > > > -- > > > > 2.17.1 > > > > > > > > > > > > -- > > Thanks > > Yafang
-- Thanks Yafang
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