Re: [PATCH -next 10/11] block, bfq: decrease 'num_groups_with_pending_reqs' earlier

[yukuai (C)]

在 2022/04/13 19:40, yukuai (C) 写道:
> 在 2022/04/13 19:28, Jan Kara 写道:
> > On Sat 05-03-22 17:12:04, Yu Kuai wrote:
> > > Currently 'num_groups_with_pending_reqs' won't be decreased when
> > > the group doesn't have any pending requests, while some child group
> > > still have pending requests. The decrement is delayed to when all the
> > > child groups doesn't have any pending requests.
> > >
> > > For example:
> > > 1) t1 issue sync io on root group, t2 and t3 issue sync io on the same
> > > child group. num_groups_with_pending_reqs is 2 now.
> > > 2) t1 stopped, num_groups_with_pending_reqs is still 2. io from t2 and
> > > t3 still can't be handled concurrently.
> > >
> > > Fix the problem by decreasing 'num_groups_with_pending_reqs'
> > > immediately upon the weights_tree removal of last bfqq of the group.
> > >
> > > Signed-off-by: Yu Kuai <yukuai3@xxxxxxxxxx>
> >
> > So I'd find the logic easier to follow if you completely removed
> > entity->in_groups_with_pending_reqs and did updates of
> > bfqd->num_groups_with_pending_reqs like:
> >
> >     if (!bfqg->num_entities_with_pending_reqs++)
> >         bfqd->num_groups_with_pending_reqs++;
> Hi,
>
> Indeed, this is an excellent idle, and much better than the way I did.
>
> Thanks,
> Kuai
>
> > and similarly on the remove side. And there would we literally two places
> > (addition & removal from weight tree) that would need to touch these
> > counters. Pretty obvious and all can be done in patch 9.
> >
> >                                 Honza
Hi, Jan

I think with this change, we can count root_group while activating bfqqs
that are under root_group, thus there is no need to modify
for_each_entity(or fake bfq_sched_data) any more.

The special case is that weight racing bfqqs are not inserted into
weights tree, and I think this can be handled by adding a fake
bfq_weight_counter for such bfqqs.

What do you think ?

Kuai