linux-kernel - Re: [PATCH -next 02/11] block, bfq: apply news apis where root group is not expected

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Date:   Wed, 13 Apr 2022 12:59:32 +0200
From:   Jan Kara <jack@...e.cz>
To:     Yu Kuai <yukuai3@...wei.com>
Cc:     tj@...nel.org, axboe@...nel.dk, paolo.valente@...aro.org,
        jack@...e.cz, cgroups@...r.kernel.org, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, yi.zhang@...wei.com
Subject: Re: [PATCH -next 02/11] block, bfq: apply news apis where root group
 is not expected

On Wed 13-04-22 11:50:44, Jan Kara wrote:
> On Sat 05-03-22 17:11:56, Yu Kuai wrote:
> > 'entity->sched_data' is set to parent group's sched_data, thus it's NULL
> > for root group. And for_each_entity() is used widely to access
> > 'entity->sched_data', thus aplly news apis if root group is not
>                              ^^ apply
> 
> > expected. Prepare to count root group into 'num_groups_with_pending_reqs'.
> > 
> > Signed-off-by: Yu Kuai <yukuai3@...wei.com>
> > ---
> >  block/bfq-iosched.c |  2 +-
> >  block/bfq-iosched.h | 22 ++++++++--------------
> >  block/bfq-wf2q.c    | 10 +++++-----
> >  3 files changed, 14 insertions(+), 20 deletions(-)
> > 
> > diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
> > index 69ddf6b0f01d..3bc7a7686aad 100644
> > --- a/block/bfq-iosched.c
> > +++ b/block/bfq-iosched.c
> > @@ -4393,7 +4393,7 @@ void bfq_bfqq_expire(struct bfq_data *bfqd,
> >  	 * service with the same budget.
> >  	 */
> >  	entity = entity->parent;
> > -	for_each_entity(entity)
> > +	for_each_entity_not_root(entity)
> >  		entity->service = 0;
> >  }
> 
> So why is it a problem to clear the service for root cgroup here?
> 
> > diff --git a/block/bfq-wf2q.c b/block/bfq-wf2q.c
> > index f8eb340381cf..c4cb935a615a 100644
> > --- a/block/bfq-wf2q.c
> > +++ b/block/bfq-wf2q.c
> > @@ -815,7 +815,7 @@ void bfq_bfqq_served(struct bfq_queue *bfqq, int served)
> >  		bfqq->service_from_wr += served;
> >  
> >  	bfqq->service_from_backlogged += served;
> > -	for_each_entity(entity) {
> > +	for_each_entity_not_root(entity) {
> >  		st = bfq_entity_service_tree(entity);
> 
> Hum, right so how come this was not crashing? Because entity->sched_data is
> indeed NULL for bfqd->root_group->entity and so bfq_entity_service_tree()
> returned some bogus pointer? Similarly for the cases you are changing
> below?

Oh, I see now. Because for_each_entity() currently does not iterate through
root cgroup because it has root_group->my_entity set to NULL and thus as a
result immediate children of root_group will have their parent set to NULL
as well.

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR