linux-kernel - Re: [PATCH 1/4] sched/fair: free allocated memory on error in alloc_fair_sched

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Date:   Wed, 2 Aug 2023 15:01:05 +0800
From:   Aaron Lu <aaron.lu@...el.com>
To:     Chen Yu <yu.c.chen@...el.com>
CC:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Daniel Jordan <daniel.m.jordan@...cle.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        "Valentin Schneider" <vschneid@...hat.com>,
        Tim Chen <tim.c.chen@...el.com>,
        "Nitin Tekchandani" <nitin.tekchandani@...el.com>,
        Waiman Long <longman@...hat.com>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/4] sched/fair: free allocated memory on error in
 alloc_fair_sched_group()

On Wed, Jul 19, 2023 at 10:13:55AM +0800, Aaron Lu wrote:
> On Tue, Jul 18, 2023 at 11:13:12PM +0800, Chen Yu wrote:
> > On 2023-07-18 at 21:41:17 +0800, Aaron Lu wrote:
> > > There is one struct cfs_rq and one struct se on each cpu for a taskgroup
> > > and when allocation for tg->cfs_rq[X] failed, the already allocated
> > > tg->cfs_rq[0]..tg->cfs_rq[X-1] should be freed. The same for tg->se.
> > > 
> > > Signed-off-by: Aaron Lu <aaron.lu@...el.com>
> > > ---
> > >  kernel/sched/fair.c | 23 ++++++++++++++++-------
> > >  1 file changed, 16 insertions(+), 7 deletions(-)
> > > 
> > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > > index a80a73909dc2..0f913487928d 100644
> > > --- a/kernel/sched/fair.c
> > > +++ b/kernel/sched/fair.c
> > > @@ -12443,10 +12443,10 @@ int alloc_fair_sched_group(struct task_group *tg, struct task_group *parent)
> > >  
> > >  	tg->cfs_rq = kcalloc(nr_cpu_ids, sizeof(cfs_rq), GFP_KERNEL);
> > >  	if (!tg->cfs_rq)
> > > -		goto err;
> > > +		return 0;
> > >  	tg->se = kcalloc(nr_cpu_ids, sizeof(se), GFP_KERNEL);
> > >  	if (!tg->se)
> > > -		goto err;
> > > +		goto err_free_rq_pointer;
> > >  
> > >  	tg->shares = NICE_0_LOAD;
> > >  
> > > @@ -12456,12 +12456,12 @@ int alloc_fair_sched_group(struct task_group *tg, struct task_group *parent)
> > >  		cfs_rq = kzalloc_node(sizeof(struct cfs_rq),
> > >  				      GFP_KERNEL, cpu_to_node(i));
> > >  		if (!cfs_rq)
> > > -			goto err;
> > > +			goto err_free;
> > >  
> > >  		se = kzalloc_node(sizeof(struct sched_entity_stats),
> > >  				  GFP_KERNEL, cpu_to_node(i));
> > >  		if (!se)
> > > -			goto err_free_rq;
> > > +			goto err_free;
> > >  
> > >  		init_cfs_rq(cfs_rq);
> > >  		init_tg_cfs_entry(tg, cfs_rq, se, i, parent->se[i]);
> > > @@ -12470,9 +12470,18 @@ int alloc_fair_sched_group(struct task_group *tg, struct task_group *parent)
> > >  
> > >  	return 1;
> > >  
> > > -err_free_rq:
> > > -	kfree(cfs_rq);
> > > -err:
> > > +err_free:
> > > +	for_each_possible_cpu(i) {
> > > +		kfree(tg->cfs_rq[i]);
> > > +		kfree(tg->se[i]);
> > > +
> > > +		if (!tg->cfs_rq[i] && !tg->se[i])
> > > +			break;
> > > +	}
> > > +	kfree(tg->se);
> > > +err_free_rq_pointer:
> > > +	kfree(tg->cfs_rq);
> > > +
> > Not sure if I overlooked, if alloc_fair_sched_group() fails in sched_create_group(),
> > would sched_free_group()->free_fair_sched_group() do the cleanup?
> 
> You are right, I overlooked... Thanks for pointing this out.

While preparing v2, one thing still looks strange in the existing code:

int alloc_fair_sched_group(struct task_group *tg, struct task_group *parent)
{
	... ...

	for_each_possible_cpu(i) {
		cfs_rq = kzalloc_node(sizeof(struct cfs_rq),
				      GFP_KERNEL, cpu_to_node(i));
		if (!cfs_rq)
			goto err;

		se = kzalloc_node(sizeof(struct sched_entity_stats),
				  GFP_KERNEL, cpu_to_node(i));
		if (!se)
			goto err_free_rq;
			~~~~~~~~~~~~~~~~

Since free_fair_sched_group() will take care freeing these memories on
error path, it looks unnecessary to do this free for a random cfs_rq
here. Or do I miss something again...?

		init_cfs_rq(cfs_rq);
		init_tg_cfs_entry(tg, cfs_rq, se, i, parent->se[i]);
		init_entity_runnable_average(se);
	}

	return 1;

err_free_rq:
	kfree(cfs_rq);
err:
	return 0;
}

I plan to change it to this:

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index a80a73909dc2..ef2618ad26eb 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -12461,7 +12461,7 @@ int alloc_fair_sched_group(struct task_group *tg, struct task_group *parent)
 		se = kzalloc_node(sizeof(struct sched_entity_stats),
 				  GFP_KERNEL, cpu_to_node(i));
 		if (!se)
-			goto err_free_rq;
+			goto err;
 
 		init_cfs_rq(cfs_rq);
 		init_tg_cfs_entry(tg, cfs_rq, se, i, parent->se[i]);
@@ -12470,8 +12470,6 @@ int alloc_fair_sched_group(struct task_group *tg, struct task_group *parent)
 
 	return 1;
 
-err_free_rq:
-	kfree(cfs_rq);
 err:
 	return 0;
 }