Messages in this thread | | | Subject | Re: [PATCH -next 2/2] block, bfq: make bfq_has_work() more accurate | From | "yukuai (C)" <> | Date | Fri, 13 May 2022 09:08:13 +0800 |
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在 2022/05/13 1:10, Jan Kara 写道: > On Thu 12-05-22 09:30:16, yukuai (C) wrote: >> On 2022/05/11 22:08, Jan Kara wrote: >>> On Tue 10-05-22 21:16:29, Yu Kuai wrote: >>>> bfq_has_work() is using busy_queues currently, which is not accurate >>>> because bfq_queue is busy doesn't represent that it has requests. Since >>>> bfqd aready has a counter 'queued' to record how many requests are in >>>> bfq, use it instead of busy_queues. >>>> >>>> Noted that bfq_has_work() can be called with 'bfqd->lock' held, thus the >>>> lock can't be held in bfq_has_work() to protect 'bfqd->queued'. >>>> >>>> Signed-off-by: Yu Kuai <yukuai3@huawei.com> >>> >>> So did you find this causing any real problem? Because bfq queue is >>> accounted among busy queues once bfq_add_bfqq_busy() is called. And that >>> happens once a new request is inserted into the queue so it should be very >>> similar to bfqd->queued. >>> >>> Honza >> >> Hi, >> >> The related problem is described here: >> >> https://lore.kernel.org/all/20220510112302.1215092-1-yukuai3@huawei.com/ >> >> The root cause of the panic is a linux-block problem, however, it can >> be bypassed if bfq_has_work() is accurate. On the other hand, >> unnecessary run_work will be triggered if bfqq stays busy: >> >> __blk_mq_run_hw_queue >> __blk_mq_sched_dispatch_requests >> __blk_mq_do_dispatch_sched >> if (!bfq_has_work()) >> break; >> blk_mq_delay_run_hw_queues -> run again after 3ms > > Ah, I see. So it is the other way around than I thought. Due to idling > bfq_tot_busy_queues() can be greater than 0 even if there are no requests > to dispatch. Indeed. OK, the patch makes sense. But please use WRITE_ONCE > for the updates of bfqd->queued. Otherwise the READ_ONCE does not really > make sense (it can still result in some bogus value due to compiler > optimizations on the write side).
Thanks for you adivce, I'll send a new version.
Kuai
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