Re: [PATCH] sched/core: Do not migrate ineligible tasks in sched_balance_rq()

[Peter Zijlstra <peterz@infradead.org>]

On Thu, Nov 28, 2024 at 07:30:37PM +0800, Hao Jia wrote:
> 
> 
> On 2024/11/28 17:19, Peter Zijlstra wrote:
> > On Thu, Nov 28, 2024 at 04:48:58PM +0800, Hao Jia wrote:
> > > From: Hao Jia <jiahao1@lixiang.com>
> > > 
> > > When the PLACE_LAG scheduling feature is enabled, 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.
> > 
> > This is not accurate, it will be eleigible, irrespective of lag, if
> > there are no other tasks. I think your patch tries to do this, but I'm
> > fairly sure it got it wrong.
> 
> Thank you for your reply. The expression in my commit message is inaccurate,
> and I will correct it in the patch v2. If I understand correctly, a task
> meeting the following conditions:
> 
>  sched_feat(PLACE_LAG) && cfs_rq->nr_running && !entity_eligible(cfs_rq,
> &p->se),
> 
> will remain ineligible both before and after migration.
> 
> If I am wrong, please correct me. Thank you!

Problem is you're checking the wrong nr_running. 

> > > @@ -9358,13 +9358,14 @@ static inline int migrate_degrades_locality(struct task_struct *p,
> > >   static
> > >   int can_migrate_task(struct task_struct *p, struct lb_env *env)
> > >   {
> > > +	struct cfs_rq *cfs_rq = task_cfs_rq(p);

This is task's current cfs_rq. What you're interested in is destination
cfs_rq. If the destination is empty, then lag is irrelevant.

You want something like:

#if CONFIG_FAIR_GROUP_SCHED
	struct task_group *tg = task_group(p);
	struct cfs_rq *dst_cfs_rq = tg->cfs_rq[env->dst_cpu];
#else
	struct cfs_rq = &env->dst_rq->cfs_rq;
#endif


Also, please add benchmark details that show this actually makes a
difference.

Notably we keep rq->cfs_tasks in MRU order; most recently ran task is
head and balancing takes from the tail, the task longest not ran.

The task longest not ran should have build up eligibility.