On Tue 26-04-22 16:27:46, yukuai (C) wrote: > 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? Well, I don't think so. Because slow disks don't tend to do a lot of internal scheduling (or have deep IO queues for that matter). Also note that generally bfq_select_queue() will not even expire a queue (despite it not having any requests to dispatch) when we should not dispatch other requests to maintain service guarantees. So I think service guarantees will be generally preserved. Obviously I could be wrong, we we will not know until we try it :). Honza -- Jan Kara <jack@xxxxxxxx> SUSE Labs, CR
