Re: [PATCH v2 5/6] sched/rt: Better manage pushing unfit tasks on wakeup

[Qais Yousef <qais.yousef@arm.com>]

On 02/24/20 11:40, Pavan Kondeti wrote:
> Hi Qais,
> 
> On Sun, Feb 23, 2020 at 06:40:00PM +0000, Qais Yousef wrote:
> > On wakeup, if a task doesn't fit the CPU it is running on (due to its
> > uclamp_min value), then we trigger the push mechanism to try to find a
> > more suitable CPU.
> > 
> > But the logic introduced in commit 804d402fb6f6 ("sched/rt: Make RT capacity-aware")
> > was incomplete. If the rq isn't overloaded, push_rt_task() will bail out
> > immediately.
> > 
> > Steve suggested using the overloaded flag to force the push, but as
> > Pavan pointed out this could cause a lot of unnecessary IPIs in case of
> > HAVE_RT_PUSH_IPI.
> > 
> > To still allow pushing unfitting task ASAP, but without causing a lot of
> > disturbance in case this is not possible (no available CPU that is
> > running at a lower priority level), introduce a new rt_nr_unfitting in
> > struct rt_rq and use that to manage how hard we try to push an unfitting
> > task in push_rt_task().
> > 
> 
> The 1-4 patches in this series are looking good to me.
> 
> At this point (after applying 4 patches), removing rt_task_fits_capacity()
> check from switched_to_rt() and task_woken_rt() would be sufficient, I think.
> i.e no changes to push/pull logic and we have a fallback for wakeup time cpu
> selection.
> 
> It is not clear what you meant by pushing the unfit task ASAP. A running
> task on a little CPU can not be pushed to BIG CPU. That would require waking
> a migration task to do the migration. The other problem is if CPU has more
> than 2 tasks (excluding running task) which one to be pushed. Are you trying
> to solve this problem?

Not that's not the problem I'm trying to solve.

Without this patch, spawning 2 tasks in Juno, I see a delay in one of the tasks
to move to a big CPU, despite the CPU not having any tasks running on it.

> 
> 
> > If the task is pinned to a single CPU, we won't inc rt_nr_unfitting,
> > hence skipping the push in this case.
> > 
> > Also there's no need to force a push on switched_to_rt(). On the next
> > wakeup we should handle it which should suffice.
> > 
> > Fixes: 804d402fb6f6 ("sched/rt: Make RT capacity-aware")
> > LINK: https://lore.kernel.org/lkml/20200221080701.GF28029@codeaurora.org/
> > Signed-off-by: Qais Yousef <qais.yousef@arm.com>
> > ---
> >  kernel/sched/rt.c    | 100 +++++++++++++++++++++++++++++++++++++++----
> >  kernel/sched/sched.h |   3 ++
> >  2 files changed, 95 insertions(+), 8 deletions(-)
> > 
> > diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
> > index 9ae8a9fabe8b..b35e49cdafcc 100644
> > --- a/kernel/sched/rt.c
> > +++ b/kernel/sched/rt.c
> > @@ -11,6 +11,7 @@ int sched_rr_timeslice = RR_TIMESLICE;
> >  int sysctl_sched_rr_timeslice = (MSEC_PER_SEC / HZ) * RR_TIMESLICE;
> >  
> >  static int do_sched_rt_period_timer(struct rt_bandwidth *rt_b, int overrun);
> > +static bool rt_task_fits_capacity(struct task_struct *p, int cpu);
> >  
> >  struct rt_bandwidth def_rt_bandwidth;
> >  
> > @@ -313,6 +314,27 @@ static void update_rt_migration(struct rt_rq *rt_rq)
> >  	}
> >  }
> >  
> > +#ifdef CONFIG_UCLAMP_TASK
> > +static void inc_rt_unfit_tasks(struct task_struct *p, struct rt_rq *rt_rq)
> > +{
> > +	int cpu = cpu_of(rq_of_rt_rq(rt_rq));
> > +
> > +	if (!rt_task_fits_capacity(p, cpu))
> > +		rt_rq->rt_nr_unfit++;
> > +}
> > +
> > +static void dec_rt_unfit_tasks(struct task_struct *p, struct rt_rq *rt_rq)
> > +{
> > +	int cpu = cpu_of(rq_of_rt_rq(rt_rq));
> > +
> > +	if (!rt_task_fits_capacity(p, cpu))
> > +		rt_rq->rt_nr_unfit--;
> > +}
> > +#else
> > +static void inc_rt_unfit_tasks(struct task_struct *p, struct rt_rq *rt_rq) {}
> > +static void dec_rt_unfit_tasks(struct task_struct *p, struct rt_rq *rt_rq) {}
> > +#endif
> > +
> >  static void inc_rt_migration(struct sched_rt_entity *rt_se, struct rt_rq *rt_rq)
> >  {
> >  	struct task_struct *p;
> > @@ -324,9 +346,17 @@ static void inc_rt_migration(struct sched_rt_entity *rt_se, struct rt_rq *rt_rq)
> >  	rt_rq = &rq_of_rt_rq(rt_rq)->rt;
> >  
> >  	rt_rq->rt_nr_total++;
> > -	if (p->nr_cpus_allowed > 1)
> > +	if (p->nr_cpus_allowed > 1) {
> >  		rt_rq->rt_nr_migratory++;
> >  
> > +		/*
> > +		 * The task is dequeued and queue again on set_cpus_allowed(),
> > +		 * so we can't end up with a unbalanced inc/dec if
> > +		 * p->nr_cpus_allowed has changed.
> > +		 */
> > +		inc_rt_unfit_tasks(p, rt_rq);
> > +	}
> > +
> >  	update_rt_migration(rt_rq);
> >  }
> >  
> > @@ -341,12 +371,29 @@ static void dec_rt_migration(struct sched_rt_entity *rt_se, struct rt_rq *rt_rq)
> >  	rt_rq = &rq_of_rt_rq(rt_rq)->rt;
> >  
> >  	rt_rq->rt_nr_total--;
> > -	if (p->nr_cpus_allowed > 1)
> > +	if (p->nr_cpus_allowed > 1) {
> >  		rt_rq->rt_nr_migratory--;
> >  
> > +		/*
> > +		 * The task is dequeued and queue again on set_cpus_allowed(),
> > +		 * so we can't end up with a unbalanced inc/dec if
> > +		 * p->nr_cpus_allowed has changed.
> > +		 */
> > +		dec_rt_unfit_tasks(p, rt_rq);
> > +	}
> > +
> 
> When uclamp values are changed via cgroups or global sysctl knobs, we don't
> enqueue/dequeue all tasks similar to sched_setattr. So a task that was fit
> at enqueue time can become unfit if uclamp values are changed in between.

Hmm good point.

> 
> >  	update_rt_migration(rt_rq);
> >  }
> >  
> > +static inline int has_unfit_tasks(struct rq *rq)
> > +{
> > +#ifdef CONFIG_UCLAMP_TASK
> > +	return rq->rt.rt_nr_unfit;
> > +#else
> > +	return 0;
> > +#endif
> > +}
> > +
> >  static inline int has_pushable_tasks(struct rq *rq)
> >  {
> >  	return !plist_head_empty(&rq->rt.pushable_tasks);
> > @@ -1862,8 +1909,9 @@ static int push_rt_task(struct rq *rq)
> >  	struct task_struct *next_task;
> >  	struct rq *lowest_rq;
> >  	int ret = 0;
> > +	bool fit;
> >  
> > -	if (!rq->rt.overloaded)
> > +	if (!rq->rt.overloaded && !has_unfit_tasks(rq))
> >  		return 0;
> >  
> 
> When there is one unfit RT task, are we setting overloaded anywhere due
> to fitness check? I don't see that in this patch.

No. I abandoned tinkering with the overloaded flag, it'd make things more
complex than I'd like them to.

> Even if we set overload condition, we can't push the running task.

This gives me the desired effect. I do see the task moving to the right CPU in
2 RT tasks test. Without it, if an RT task was spawned on little CPU I could
see a big delay for it to move to a big CPU although the big CPU is free.

> 
> >  	next_task = pick_next_pushable_task(rq);
> > @@ -1874,12 +1922,21 @@ static int push_rt_task(struct rq *rq)
> >  	if (WARN_ON(next_task == rq->curr))
> >  		return 0;
> >  
> > +	/*
> > +	 * The rq could be overloaded because it has unfitting task, if that's
> > +	 * the case then we need to try harder to find a better fitting CPU.
> > +	 */
> > +	fit = rt_task_fits_capacity(next_task, cpu_of(rq));
> > +
> >  	/*
> >  	 * It's possible that the next_task slipped in of
> >  	 * higher priority than current. If that's the case
> >  	 * just reschedule current.
> > +	 *
> > +	 * Unless next_task doesn't fit in this cpu, then continue with the
> > +	 * attempt to push it.
> >  	 */
> > -	if (unlikely(next_task->prio < rq->curr->prio)) {
> > +	if (unlikely(next_task->prio < rq->curr->prio && fit)) {
> >  		resched_curr(rq);
> >  		return 0;
> >  	}
> > @@ -1922,6 +1979,35 @@ static int push_rt_task(struct rq *rq)
> >  		goto retry;
> >  	}
> >  
> > +	/*
> > +	 * Bail out if the task doesn't fit on either CPUs.
> > +	 *
> > +	 * Unless..
> > +	 *
> > +	 * * The rq is already overloaded, then push anyway.
> > +	 *
> > +	 * * The priority of next_task is higher than current, then we
> > +	 *   resched_curr(). We forced skipping this condition above if the rq
> > +	 *   was overloaded but the task didn't fit.
> > +	 */
> > +	if (!fit && !rt_task_fits_capacity(next_task, cpu_of(lowest_rq))) {
> > +
> > +		/*
> > +		 * If the system wasn't overloaded, then pretend we didn't run.
> > +		 */
> > +		if (!rq->rt.overloaded)
> > +			goto out_unlock;
> > +
> > +		/*
> > +		 * If the system is overloaded, we forced skipping this
> > +		 * condition, so re-evaluate it.
> > +		 */
> > +		if (unlikely(next_task->prio < rq->curr->prio)) {
> > +			resched_curr(rq);
> > +			goto out_unlock;
> > +		}
> > +	}
> > +
> >  	deactivate_task(rq, next_task, 0);
> >  	set_task_cpu(next_task, lowest_rq->cpu);
> >  	activate_task(lowest_rq, next_task, 0);
> > @@ -1929,6 +2015,7 @@ static int push_rt_task(struct rq *rq)
> >  
> >  	resched_curr(lowest_rq);
> >  
> > +out_unlock:
> >  	double_unlock_balance(rq, lowest_rq);
> >  
> >  out:
> > @@ -2297,10 +2384,7 @@ static void switched_to_rt(struct rq *rq, struct task_struct *p)
> >  	 */
> >  	if (task_on_rq_queued(p) && rq->curr != p) {
> >  #ifdef CONFIG_SMP
> > -		bool need_to_push = rq->rt.overloaded ||
> > -				    !rt_task_fits_capacity(p, cpu_of(rq));
> > -
> > -		if (p->nr_cpus_allowed > 1 && need_to_push)
> > +		if (p->nr_cpus_allowed > 1 && rq->rt.overloaded)
> >  			rt_queue_push_tasks(rq);
> 
> Right. What about the check in task_woken_rt()? We should remove it
> from there too.

We could do, temporarily, to get these fixes into 5.6. But I do think
select_task_rq_rt() doesn't do a good enough job into pushing unfit tasks to
the right CPUs.

I don't understand the reasons behind your objection. It seems you think that
select_task_rq_rt() should be enough, but not AFAICS. Can you be a bit more
detailed please?

FWIW, here's a screenshot of what I see

	https://imgur.com/a/peV27nE

After the first activation, select_task_rq_rt() fails to find the right CPU
(due to the same move all tasks to the cpumask_fist()) - but when the task
wakes up on 4, the logic I put causes it to migrate to CPU2, which is the 2nd
big core. CPU1 and CPU2 are the big cores on Juno.

Now maybe we should fix select_task_rq_rt() to better balance tasks, but not
sure how easy is that.

Steve suggested the overload idea, so I assumed it's okay. But given the IPI
issue you highlighted, I thought better not make overloaded conditions more
complex and just a take a simpler approach which I think should be fine - bar
the uclamp caveat you highlighted. Which should be solvable..

Thanks

--
Qais Yousef

> 
> >  #endif /* CONFIG_SMP */
> >  		if (p->prio < rq->curr->prio && cpu_online(cpu_of(rq)))
> > diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> > index 1a88dc8ad11b..7dea81ccd49a 100644
> > --- a/kernel/sched/sched.h
> > +++ b/kernel/sched/sched.h
> > @@ -603,6 +603,9 @@ struct rt_rq {
> >  #ifdef CONFIG_SMP
> >  	unsigned long		rt_nr_migratory;
> >  	unsigned long		rt_nr_total;
> > +#ifdef CONFIG_UCLAMP_TASK
> > +	unsigned long		rt_nr_unfit;
> > +#endif
> >  	int			overloaded;
> >  	struct plist_head	pushable_tasks;
> >  
> > -- 
> > 2.17.1
> > 
> 
> -- 
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