Re: [PATCH -next v2 2/5] block, bfq: add fake weight_counter for weight-raised queue

[yukuai (C)]

在 2022/04/26 0:16, Jan Kara 写道:
> Hello!
>
> On Mon 25-04-22 21:34:16, yukuai (C) wrote:
> > 在 2022/04/25 17:48, Jan Kara 写道:
> > > On Sat 16-04-22 17:37:50, Yu Kuai wrote:
> > > > Weight-raised queue is not inserted to weights_tree, which makes it
> > > > impossible to track how many queues have pending requests through
> > > > weights_tree insertion and removel. This patch add fake weight_counter
> > > > for weight-raised queue to do that.
> > > >
> > > > Signed-off-by: Yu Kuai <yukuai3@xxxxxxxxxx>
> > >
> > > This is a bit hacky. I was looking into a better place where to hook to
> > > count entities in a bfq_group with requests and I think bfq_add_bfqq_busy()
> > > and bfq_del_bfqq_busy() are ideal for this. It also makes better sense
> > > conceptually than hooking into weights tree handling.
> >
> > bfq_del_bfqq_busy() will be called when all the reqs in the bfqq are
> > dispatched, however there might still some reqs are't completed yet.
> >
> > Here what we want to track is how many bfqqs have pending reqs,
> > specifically if the bfqq have reqs are't complted.
> >
> > Thus I think bfq_del_bfqq_busy() is not the right place to do that.
>
> Yes, I'm aware there will be a difference. But note that bfqq can stay busy
> with only dispatched requests because the logic in __bfq_bfqq_expire() will
> not call bfq_del_bfqq_busy() if idling is needed for service guarantees. So
> I think using bfq_add/del_bfqq_busy() would work OK.
Hi,

I didn't think of that before. If bfqq stay busy after dispathing all
the requests, there are two other places that bfqq can clear busy:

1) bfq_remove_request(), bfqq has to insert a new req while it's not in
service.

2) bfq_release_process_ref(), user thread is gone / moved, or old bfqq
is gone due to merge / ioprio change.

I wonder, will bfq_del_bfqq_busy() be called immediately when requests
are completed? (It seems not to me...). For example, a user thread
issue a sync io just once, and it keep running without issuing new io,
then when does the bfqq clears the busy state?

Thanks,
Kuai
> 								Honza