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

[yukuai (C)]

在 2022/04/26 15:40, Jan Kara 写道:
> On Tue 26-04-22 09:49:04, yukuai (C) wrote:
> > 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.
>
> Yes and the request then would have to be dispatched or merged. Which
> generally means another bfqq from the same bfqg is currently active and
> thus this should have no impact on service guarantees we are interested in.
>
> > 2) bfq_release_process_ref(), user thread is gone / moved, or old bfqq
> > is gone due to merge / ioprio change.
>
> Yes, here there's no new IO for the bfqq so no point in maintaining any
> service guarantees to it.
>
> > 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?
>
> No, when bfqq is kept busy, it will get scheduled as in-service queue in
> the future. Then what happens depends on whether it will get more requests
> or not. But generally its busy state will get cleared once it is expired
> for other reason than preemption.

Thanks for your explanation.

I think in normal case using bfq_add/del_bfqq_busy() if fine.

There is one last situation that I'm worried: If some disk are very
slow that the dispatched reqs are not completed when the bfqq is
rescheduled as in-service queue, and thus busy state can be cleared
while reqs are not completed.

Using bfq_del_bfqq_busy() will change behaviour in this specail case,
do you think service guarantees will be broken?

Thanks,
Kuai