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

From: Yafang Shao
Date: Mon Jun 14 2021 - 12:01:41 EST

On Mon, Jun 14, 2021 at 10:21 PM Vincent Guittot
<vincent.guittot@xxxxxxxxxx> wrote:
>
> On Mon, 14 Jun 2021 at 12:23, Yafang Shao <laoar.shao@xxxxxxxxx> wrote:
> >
> > On Wed, Jun 2, 2021 at 8:37 PM Vincent Guittot
> > <vincent.guittot@xxxxxxxxxx> wrote:
> > >
> > > On Wed, 2 Jun 2021 at 14:26, Yafang Shao <laoar.shao@xxxxxxxxx> 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);
>

Right.

> >
> > 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)
>

Sure

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

Thanks for your information.
Seems that's a similar issue.

>
> > + raw_spin_unlock_irqrestore(&busiest->lock,
> > + flags);
> > + goto out_one_pinned;
>
> goto out;
> is enough because the the task is not pinned
>

Sure.

>
> > + }
> > +
> > /* 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
>

Sure.

I will update the patch.

>
> >
> > >
> > >
> > > >
> > > > 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@xxxxxxxxx>
> > > > ---
> > > > 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