Re: [PATCH 5/8] sched,cfs: use explicit cfs_rq of parent se helper

From: Rik van Riel <riel@surriel.com>
To: Dietmar Eggemann <dietmar.eggemann@arm.com>, peterz@infradead.org
Cc: mingo@redhat.com, linux-kernel@vger.kernel.org,
	kernel-team@fb.com, morten.rasmussen@arm.com, tglx@linutronix.de,
	dietmar.eggeman@arm.com, mgorman@techsingularity.com,
	vincent.guittot@linaro.org
Subject: Re: [PATCH 5/8] sched,cfs: use explicit cfs_rq of parent se helper
Date: Thu, 20 Jun 2019 12:29:22 -0400	[thread overview]
Message-ID: <c5c73fd374ed952eedc46a89af294e1d521609b2.camel@surriel.com> (raw)
In-Reply-To: <55c0dc01-24b2-bb7f-6ceb-b65c52f7b46b@arm.com>

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On Thu, 2019-06-20 at 18:23 +0200, Dietmar Eggemann wrote:
> On 6/12/19 9:32 PM, Rik van Riel wrote:
> > Use an explicit "cfs_rq of parent sched_entity" helper in a few
> > strategic places, where cfs_rq_of(se) may no longer point at the
> > right runqueue once we flatten the hierarchical cgroup runqueues.
> > 
> > No functional change.
> > 
> > Signed-off-by: Rik van Riel <riel@surriel.com>
> > ---
> >  kernel/sched/fair.c | 17 +++++++++++++----
> >  1 file changed, 13 insertions(+), 4 deletions(-)
> > 
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index dcc521d251e3..c6ede2ecc935 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -275,6 +275,15 @@ static inline struct cfs_rq
> > *group_cfs_rq(struct sched_entity *grp)
> >  	return grp->my_q;
> >  }
> >  
> > +/* runqueue owned by the parent entity */
> > +static inline struct cfs_rq *group_cfs_rq_of_parent(struct
> > sched_entity *se)
> > +{
> > +	if (se->parent)
> > +		return group_cfs_rq(se->parent);
> > +
> > +	return &cfs_rq_of(se)->rq->cfs;
> > +}
> > +
> >  static inline bool list_add_leaf_cfs_rq(struct cfs_rq *cfs_rq)
> >  {
> >  	struct rq *rq = rq_of(cfs_rq);
> > @@ -3298,7 +3307,7 @@ static inline int
> > propagate_entity_load_avg(struct sched_entity *se)
> >  
> >  	gcfs_rq->propagate = 0;
> >  
> > -	cfs_rq = cfs_rq_of(se);
> > +	cfs_rq = group_cfs_rq_of_parent(se);
> >  
> >  	add_tg_cfs_propagate(cfs_rq, gcfs_rq->prop_runnable_sum);
> >  
> > @@ -7779,7 +7788,7 @@ static void update_cfs_rq_h_load(struct
> > cfs_rq *cfs_rq)
> >  
> >  	WRITE_ONCE(cfs_rq->h_load_next, NULL);
> >  	for_each_sched_entity(se) {
> > -		cfs_rq = cfs_rq_of(se);
> > +		cfs_rq = group_cfs_rq_of_parent(se);
> 
> Why do you change this here? task_se_h_load() calls
> update_cfs_rq_h_load() with cfs_rq = group_cfs_rq_of_parent(se)
> because
> the task might not be on the cfs_rq yet.

Because patch 6 points cfs_rq_of(se) at the CPU's top level
cfs_rq for every task se ...

... but I guess since I have not changed where the cfs_rq_of
points for cgroup sched_entities, this change is not necessary
at this time, and I should be able to go without it, in this
function.

> But inside update_cfs_rq_h_load() the first se is derived from
> cfs_rq->tg->se[cpu_of(rq)] so in the first for_each_sched_entity()
> loop
> we should still start with group_cfs_rq() (se->my_q) ?
> 
> The system doesn't barf with these two WARN_ON's in.
> 
> @@ -7663,12 +7673,17 @@ static void update_cfs_rq_h_load(struct
> cfs_rq
> *cfs_rq)
>         unsigned long now = jiffies;
>         unsigned long load;
> 
> +       WARN_ON(se && (se != group_cfs_rq(se)->tg->se[cpu_of(rq)]));
> +
>         if (cfs_rq->last_h_load_update == now)
>                 return;
> 
>         WRITE_ONCE(cfs_rq->h_load_next, NULL);
>         for_each_sched_entity(se) {
>                 cfs_rq = group_cfs_rq_of_parent(se);
> +
> +               WARN_ON(se != group_cfs_rq(se)->tg->se[cpu_of(rq)]);
> +
>                 WRITE_ONCE(cfs_rq->h_load_next, se);
>                 if (cfs_rq->last_h_load_update == now)
>                         break;
> 
> 
> [...]
> 
> 
-- 
All Rights Reversed.

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