linux-kernel - Re: [PATCH] sched,fair: use list_for_each_entry() in print_cfs

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Date:   Thu, 8 Oct 2020 20:37:10 +0800
From:   Hui Su <sh_def@....com>
To:     Dietmar Eggemann <dietmar.eggemann@....com>
Cc:     mingo@...hat.com, linux-kernel@...r.kernel.org,
        peterz@...radead.org, juri.lelli@...hat.com
Subject: Re: [PATCH] sched,fair: use list_for_each_entry() in
 print_cfs_stats()

On Tue, Sep 29, 2020 at 11:56:42AM +0200, Dietmar Eggemann wrote:
> On 25/09/2020 21:10, Hui Su wrote:
> > Macro for_each_leaf_cfs_rq_safe() use list_for_each_entry_safe(),
> > which can against removal of list entry, but we only
> > print the cfs_rq data and won't remove the list entry in
> > print_cfs_stats().
> > 
> > Thus, add macro for_each_leaf_cfs_rq() based on
> > list_for_each_entry(), and use for_each_leaf_cfs_rq() in
> > print_cfs_stats().
> > 
> > Signed-off-by: Hui Su <sh_def@....com>
> > ---
> >  kernel/sched/fair.c | 11 ++++++++---
> >  1 file changed, 8 insertions(+), 3 deletions(-)
> > 
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index 1a68a0536add..d40dfb4349b0 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -391,11 +391,16 @@ static inline void assert_list_leaf_cfs_rq(struct rq *rq)
> >  	SCHED_WARN_ON(rq->tmp_alone_branch != &rq->leaf_cfs_rq_list);
> >  }
> >  
> > -/* Iterate thr' all leaf cfs_rq's on a runqueue */
> > +/* Iterate thr' all leaf cfs_rq's on a runqueue safely */
> >  #define for_each_leaf_cfs_rq_safe(rq, cfs_rq, pos)			\
> >  	list_for_each_entry_safe(cfs_rq, pos, &rq->leaf_cfs_rq_list,	\
> >  				 leaf_cfs_rq_list)
> >  
> > +/* Iterate thr' all leaf cfs_rq's on a runqueue */
> > +#define for_each_leaf_cfs_rq(rq, cfs_rq)			\
> > +	list_for_each_entry(cfs_rq, &rq->leaf_cfs_rq_list,	\
> > +				 leaf_cfs_rq_list)
> > +
> >  /* Do the two (enqueued) entities belong to the same group ? */
> >  static inline struct cfs_rq *
> >  is_same_group(struct sched_entity *se, struct sched_entity *pse)
> > @@ -11185,10 +11190,10 @@ const struct sched_class fair_sched_class
> >  #ifdef CONFIG_SCHED_DEBUG
> >  void print_cfs_stats(struct seq_file *m, int cpu)
> >  {
> > -	struct cfs_rq *cfs_rq, *pos;
> > +	struct cfs_rq *cfs_rq;
> >  
> >  	rcu_read_lock();
> > -	for_each_leaf_cfs_rq_safe(cpu_rq(cpu), cfs_rq, pos)
> > +	for_each_leaf_cfs_rq(cpu_rq(cpu), cfs_rq)
> >  		print_cfs_rq(m, cpu, cfs_rq);
> >  	rcu_read_unlock();
> >  }
> 
> IMHO, for_each_leaf_cfs_rq_safe() was introduced in commit a9e7f6544b9c
> ("sched/fair: Fix O(nr_cgroups) in load balance path") and reintroduced
> again by commit 039ae8bcf7a5 ("sched/fair: Fix O(nr_cgroups) in the load
> balancing path") to prevent races between tasks running
> print_cfs_stats() and today's  __update_blocked_fair() ->
> list_del_leaf_cfs_rq(cfs_rq).
> 
> Your patch doesn't compile w/ !CONFIG_FAIR_GROUP_SCHED.

Thanks for your explanation, please ignore this change.