Re: [PATCH 2/3] sched/rt: allow pulling unfitting task

[Qais Yousef]

On 02/17/20 14:40, Pavan Kondeti wrote:
> Hi Qais,
> 
> On Fri, Feb 14, 2020 at 04:39:48PM +0000, Qais Yousef wrote:
> > When implemented RT Capacity Awareness; the logic was done such that if
> > a task was running on a fitting CPU, then it was sticky and we would try
> > our best to keep it there.
> > 
> > But as Steve suggested, to adhere to the strict priority rules of RT
> > class; allow pulling an RT task to unfitting CPU to ensure it gets a
> > chance to run ASAP. When doing so, mark the queue as overloaded to give
> > the system a chance to push the task to a better fitting CPU when a
> > chance arises.
> > 
> > Suggested-by: Steven Rostedt <rostedt@xxxxxxxxxxx>
> > Signed-off-by: Qais Yousef <qais.yousef@xxxxxxx>
> > ---
> >  kernel/sched/rt.c | 16 +++++++++++++---
> >  1 file changed, 13 insertions(+), 3 deletions(-)
> > 
> > diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
> > index 4043abe45459..0c8bac134d3a 100644
> > --- a/kernel/sched/rt.c
> > +++ b/kernel/sched/rt.c
> > @@ -1646,10 +1646,20 @@ static void put_prev_task_rt(struct rq *rq, struct task_struct *p)
> >  
> >  static int pick_rt_task(struct rq *rq, struct task_struct *p, int cpu)
> >  {
> > -	if (!task_running(rq, p) &&
> > -	    cpumask_test_cpu(cpu, p->cpus_ptr) &&
> > -	    rt_task_fits_capacity(p, cpu))
> > +	if (!task_running(rq, p) && cpumask_test_cpu(cpu, p->cpus_ptr)) {
> > +
> > +		/*
> > +		 * If the CPU doesn't fit the task, allow pulling but mark the
> > +		 * rq as overloaded so that we can push it again to a more
> > +		 * suitable CPU ASAP.
> > +		 */
> > +		if (!rt_task_fits_capacity(p, cpu)) {
> > +			rt_set_overload(rq);
> > +			rq->rt.overloaded = 1;
> > +		}
> > +
> 
> Here rq is source rq from which the task is being pulled. I can't understand
> how marking overload condition on source_rq help. Because overload condition
> gets cleared in the task dequeue path. i.e dec_rt_tasks->dec_rt_migration->
> update_rt_migration().
> 
> Also, the overload condition with nr_running=1 may not work as expected unless
> we track this overload condition (due to unfit) separately. Because a task
> can be pushed only when it is NOT running. So a task running on silver will
> continue to run there until it wakes up next time or another high prio task
> gets queued here (due to affinity).
> 
> btw, Are you testing this path by disabling RT_PUSH_IPI feature? I ask this
> because, This feature gets turned on by default in our b.L platforms and
> RT task migrations happens by the busy CPU pushing the tasks. Or are there
> any cases where we can run into pick_rt_task() even when RT_PUSH_IPI is
> enabled?

I changed the approach to set the overload at wake up now. I think I got away
without having to encode the reason in rq->rt.overload.

Steve, Pavan, if you can scrutinize this approach I'd be appreciated. It seemed
to work fine with my testing.

I'll push an updated series if this turned out okay.

Thanks

--
Qais Yousef

-->8--


When implemented RT Capacity Awareness; the logic was done such that if
a task was running on a fitting CPU, then it was sticky and we would try
our best to keep it there.

But as Steve suggested, to adhere to the strict priority rules of RT
class; allow pulling an RT task to unfitting CPU to ensure it gets a
chance to run ASAP.

To better handle the fact the task is running on unfit CPU, when it
wakes up mark it as overloaded which will cause it to be pushed to
a fitting CPU when it becomes available.

The latter change requires teaching push_rt_task() how to handle pushing
unfit task.

If the unfit task is the only pushable task, then we only force the push
if we find a fitting CPU. Otherwise we bail out.

Else if the task is higher priorirty than current task, then we
reschedule.

Else if the rq has other pushable tasks, then we push the unfitting task
anyway to reduce the pressure on the rq even if the target CPU is unfit
too.

Suggested-by: Steven Rostedt <rostedt@xxxxxxxxxxx>
Signed-off-by: Qais Yousef <qais.yousef@xxxxxxx>
---
 kernel/sched/rt.c | 52 +++++++++++++++++++++++++++++++++++++++++++----
 1 file changed, 48 insertions(+), 4 deletions(-)

diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
index 6d959be4bba0..6d92219d5733 100644
--- a/kernel/sched/rt.c
+++ b/kernel/sched/rt.c
@@ -1658,8 +1658,7 @@ static void put_prev_task_rt(struct rq *rq, struct task_struct *p)
 static int pick_rt_task(struct rq *rq, struct task_struct *p, int cpu)
 {
 	if (!task_running(rq, p) &&
-	    cpumask_test_cpu(cpu, p->cpus_ptr) &&
-	    rt_task_fits_capacity(p, cpu))
+	    cpumask_test_cpu(cpu, p->cpus_ptr))
 		return 1;
 
 	return 0;
@@ -1860,6 +1859,7 @@ 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)
 		return 0;
@@ -1872,12 +1872,21 @@ 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 they 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;
 	}
@@ -1920,6 +1929,33 @@ static int push_rt_task(struct rq *rq)
 		goto retry;
 	}
 
+	/*
+	 * Bail out if the task doesn't fit on either CPUs.
+	 *
+	 * Unless..
+	 *
+	 * * The priority of next_task is higher than current, then we
+	 *   resched_curr(). We forced skipping this condition above.
+	 *
+	 * * The rq has more tasks to push, then we probably should push anyway
+	 *   reduce the load on this rq.
+	 */
+	if (!fit && !rt_task_fits_capacity(next_task, cpu_of(lowest_rq))) {
+
+		/* we forced skipping this condition, so re-evaluate it */
+		if (unlikely(next_task->prio < rq->curr->prio)) {
+			resched_curr(rq);
+			goto out_unlock;
+		}
+
+		/*
+		 * If there are more tasks to push, then the rq is overloaded
+		 * with more than just this task, so push anyway.
+		 */
+		if (has_pushable_tasks(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);
@@ -1927,6 +1963,7 @@ static int push_rt_task(struct rq *rq)
 
 	resched_curr(lowest_rq);
 
+out_unlock:
 	double_unlock_balance(rq, lowest_rq);
 
 out:
@@ -2223,7 +2260,14 @@ static void task_woken_rt(struct rq *rq, struct task_struct *p)
 			    (rq->curr->nr_cpus_allowed < 2 ||
 			     rq->curr->prio <= p->prio);
 
-	if (need_to_push || !rt_task_fits_capacity(p, cpu_of(rq)))
+	bool fit = rt_task_fits_capacity(p, cpu_of(rq));
+
+	if (!fit && !rq->rt.overloaded) {
+		rt_set_overload(rq);
+		rq->rt.overloaded = 1;
+	}
+
+	if (need_to_push || !fit)
 		push_rt_tasks(rq);
 }
 
-- 
2.17.1