Messages in this thread Patch in this message | | | Subject | Re: [PATCH -next 2/2] block, bfq: make bfq_has_work() more accurate | From | "yukuai (C)" <> | Date | Thu, 12 May 2022 09:30:16 +0800 |
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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
Thanks, Kuai > >> --- >> block/bfq-iosched.c | 4 ++-- >> 1 file changed, 2 insertions(+), 2 deletions(-) >> >> diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c >> index 61750696e87f..1d2f8110c26b 100644 >> --- a/block/bfq-iosched.c >> +++ b/block/bfq-iosched.c >> @@ -5063,11 +5063,11 @@ static bool bfq_has_work(struct blk_mq_hw_ctx *hctx) >> struct bfq_data *bfqd = hctx->queue->elevator->elevator_data; >> >> /* >> - * Avoiding lock: a race on bfqd->busy_queues should cause at >> + * Avoiding lock: a race on bfqd->queued should cause at >> * most a call to dispatch for nothing >> */ >> return !list_empty_careful(&bfqd->dispatch) || >> - bfq_tot_busy_queues(bfqd) > 0; >> + READ_ONCE(bfqd->queued); >> } >> >> static struct request *__bfq_dispatch_request(struct blk_mq_hw_ctx *hctx) >> -- >> 2.31.1 >>
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