> Il giorno 26 apr 2022, alle ore 11:15, Jan Kara <jack@xxxxxxx> ha scritto: > > 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 :). > I have nothing to add ... You guys are getting better than me about BFQ :) Thanks, Paolo > Honza > > -- > Jan Kara <jack@xxxxxxxx> > SUSE Labs, CR
