Re: [PATCH v2] sched/core: Prioritize migrating eligible tasks in sched_balance_rq()

[Hao Jia]

On 2025/1/14 16:07, Vincent Guittot wrote:
> On Tue, 14 Jan 2025 at 04:18, Hao Jia <jiahao.kernel@xxxxxxxxx> wrote:
> > On 2025/1/14 00:40, Vincent Guittot wrote:
> > > On Mon, 13 Jan 2025 at 10:21, Hao Jia <jiahao.kernel@xxxxxxxxx> wrote:
> > > > Friendly ping...
> > > >
> > > >
> > > > On 2024/12/23 17:14, Hao Jia wrote:
> > > > > From: Hao Jia <jiahao1@xxxxxxxxxxx>
> > > > >
> > > > > When the PLACE_LAG scheduling feature is enabled and
> > > > > dst_cfs_rq->nr_queued is greater than 1, if a task is
> > > > > ineligible (lag < 0) on the source cpu runqueue, it will
> > > > > also be ineligible when it is migrated to the destination
> > > > > cpu runqueue. Because we will keep the original equivalent
> > > > > lag of the task in place_entity(). So if the task was
> > > > > ineligible before, it will still be ineligible after
> > > > > migration.
> > > > >
> > > > > So in sched_balance_rq(), we prioritize migrating eligible
> > > > > tasks, and we soft-limit ineligible tasks, allowing them
> > > > > to migrate only when nr_balance_failed is non-zero to
> > > > > avoid load-balancing trying very hard to balance the load.
> > >
> > > Could you explain why you think it's better to balance eligible tasks
> > > in priority and potentially skip a load balance ?
> >
> > In place_entity(), we maintain the task's original equivalent lag, even
> > if we migrate the task to dst_rq, this does not change its eligibility
> > attribute.
>
> Yes, but you don't answer the question why it's better to select an
> eligible task vs a non eligible task.
>
> > When there are multiple tasks on src_rq, and the dst_cpu has some
> > runnable tasks, migrating ineligible tasks to dst_rq will not allow them
> > to run. Therefore, such task migration is inefficient. We should
>
> Why is it inefficient ? load balance is about evenly balancing the
> number of tasks or the load between CPUs, it never says that the newly
> migrated task should run immediately


My initial thought is that when we need to migrate some tasks during 
load balancing, at the current point in time, migrating ineligible tasks 
to dst_cpu means they definitely cannot run there. Therefore, I prefer 
to keep them on src_cpu to reduce the overhead of dequeueing and 
enqueueing ineligible tasks.

Migrating eligible tasks to dst_cpu does not guarantee that they will 
run earlier than on src_cpu. it depends on too many factors.



> > prioritize migrating tasks that can run on dst_rq.
> >
> > In other words, migrating ineligible tasks is merely moving them to
> > another runqueue to wait until they become eligible.
>
> But I don't get why it's a problem. Migrating an eligible task might
> delay its scheduling because of its deadline vs other tasks already
> eligible on the dst_rq. Eligible and non eligible tasks are all
> runnable, it's just how much they have already run. In addition,
> migrating an eligible task will clear its positive vlag with
> DELAY_ZERO which is unfair IMO


Sorry, I'd like to ask you a question that confuses me: Why does 
migrating eligible task will clear the positive vlag?

In detach_task(), the ENQUEUE_DELAYED and DEQUEUE_SLEEP flags are not 
set, and in dequeue_entity(), eligible tasks will not set sched_delayed, 
so they will be dequeued normally with se->on_rq being 0.

Similarly, attach_task() does not set the ENQUEUE_DELAYED and 
DEQUEUE_SLEEP flags, and since se->on_rq is 0, it will not call 
requeue_delayed_entity().


Thanks,
Hao

> > > I can see an interest for idle and newly_idle load balance in order to
> > > favor fairness as tasks will become eligible but I don't see why it
> > > would be helpful if dst already has some runnable tasks. Furthermore,
> > > when a cpu is idle or newly idle, we really want to migrate a task
> > > even an non eligible one instead of possibly skipping this load
> > > balance round. With your patch, we might end up not pulling any task,
> > > increasing the nr_balance_failed and waiting next load balance
> >
> > If I understand correctly, when the destination CPU is idle, my patch
> > does not change the original behavior. it only prevents the migration of
> > ineligible tasks when dst_cfs_rq->nr_queued is greater than 1.
>
> It changes the behavior. My concern is that migrating an eligible task
> when dst rq already has runnable tasks, doesn't assure you that it
> will give any advantage to this eligible task.
>
> On an idle or newly idle cpu, any runnable tasks that will be pulled
> will immediately start to run whatever it was eligible or not on src
> cpu. In such a situation, we could consider that selecting an eligible
> task which has less running time (positive lag) than others could be
> more fair because the eligible task will immediately run. But this is
> true as long as we migrate only 1 task. If you migrate several tasks,
> an eligible task on src rq could even become ineligible quicker on dst
> cpu than on src cpu has it lost its lag