Re: [PATCH 1/1] sched: do active load balance on the new idle cpu

[Vincent Guittot <vincent.guittot@linaro.org>]

On Mon, 14 Jun 2021 at 12:23, 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.
> > 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
> >
>
> Hi Vincent,
>
> When I analyze it on my test server, I find the race really exists. For example,
>
> sensing_node-2511 [007] d... 945.351566: sched_switch:
> prev_comm=sensing_node prev_pid=2511 prev_prio=98 prev_state=S ==>
> next_comm=cat next_pid=2686 next_prio=120
> cat-2686 [007] d... 945.351569: sched_switch: prev_comm=cat
> prev_pid=2686 prev_prio=120 prev_state=R+ ==> next_comm=sensing_node
> next_pid=2512 next_prio=98
> sensing_node-2516 [004] dn.. 945.351571: sched_wakeup:
> comm=migration/7 pid=47 prio=0 target_cpu=007
> sensing_node-2512 [007] d... 945.351572: sched_switch:
> prev_comm=sensing_node prev_pid=2512 prev_prio=98 prev_state=R ==>
> next_comm=migration/7 next_pid=47 next_prio=0
> sensing_node-2516 [004] d... 945.351572: sched_switch:
> prev_comm=sensing_node prev_pid=2516 prev_prio=98 prev_state=S ==>
> next_comm=sensing_node next_pid=2502 next_prio=98
> migration/7-47 [007] d... 945.351580: sched_switch:
> prev_comm=migration/7 prev_pid=47 prev_prio=0 prev_state=S ==>
> next_comm=sensing_node next_pid=2512 next_prio=98
> sensing_node-2502 [004] d... 945.351605: sched_switch:
> prev_comm=sensing_node prev_pid=2502 prev_prio=98 prev_state=S ==>
> next_comm=cat next_pid=2686 next_prio=120
>
> When CPU4 is waking migration/7, the CFS thread 'cat' is running on
> CPU7, but then 'cat' is preempted by a RT task 'sensing_node', and
> then the migration/7 preempts the RT task.

ok the race happens between :
    if (need_active_balance(&env)) {
and
        raw_spin_rq_lock_irqsave(busiest, flags);

>
> What about below patch to improve the race ? It can't avoid the race,
> but it could reduce the race.
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 3248e24a90b0..0e8d31e17dc7 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -9794,6 +9794,20 @@ static int load_balance(int this_cpu, struct rq *this_rq,
>                                 goto out_one_pinned;
>                         }
>
> +                       /*
> +                        * There may be a race between new idle load
> balance starting

s/new idle load/load/

In fact, the same can happen during all kind of  load balance


> +                        * migration thread to pull the cfs running
> thread and the RT
> +                        * thread waking up and preempting cfs task
> before migration
> +                        * threads which then preempt the RT thread.
> +                        * We'd better do the last minute check before starting
> +                        * migration thread to avoid preempting
> latency-sensitive RT thread.
> +                        */
> +                       if (dl_task(busiest->curr) || rt_task(busiest->curr)) {

if(busiest->curr->sched_class != &fair_sched_class)

Reviewing your proposal reminded me a similar discussion with Valentin:
https://lore.kernel.org/lkml/CAKfTPtBygNcVewbb0GQOP5xxO96am3YeTZNP5dK9BxKHJJAL-g@mail.gmail.com/


> +                               raw_spin_unlock_irqrestore(&busiest->lock,
> +                                                          flags);
> +                               goto out_one_pinned;

                               goto out;
is enough because the the task is not pinned


> +                       }
> +
>                         /* Record that we found at least one task that
> could run on this_cpu */
>                         env.flags &= ~LBF_ALL_PINNED;

Your test should be moved after clearing the LBF_ALL_PINNED flag


>
> >
> >
> > >
> > > 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