From 613471549f366cdf4170b81ce0f99f3867ec4d16 Mon Sep 17 00:00:00 2001 From: Jan Kara Date: Thu, 3 Jun 2021 12:47:21 +0200 Subject: block: Do not pull requests from the scheduler when we cannot dispatch them Provided the device driver does not implement dispatch budget accounting (which only SCSI does) the loop in __blk_mq_do_dispatch_sched() pulls requests from the IO scheduler as long as it is willing to give out any. That defeats scheduling heuristics inside the scheduler by creating false impression that the device can take more IO when it in fact cannot. For example with BFQ IO scheduler on top of virtio-blk device setting blkio cgroup weight has barely any impact on observed throughput of async IO because __blk_mq_do_dispatch_sched() always sucks out all the IO queued in BFQ. BFQ first submits IO from higher weight cgroups but when that is all dispatched, it will give out IO of lower weight cgroups as well. And then we have to wait for all this IO to be dispatched to the disk (which means lot of it actually has to complete) before the IO scheduler is queried again for dispatching more requests. This completely destroys any service differentiation. So grab request tag for a request pulled out of the IO scheduler already in __blk_mq_do_dispatch_sched() and do not pull any more requests if we cannot get it because we are unlikely to be able to dispatch it. That way only single request is going to wait in the dispatch list for some tag to free. Reviewed-by: Ming Lei Signed-off-by: Jan Kara Link: https://lore.kernel.org/r/20210603104721.6309-1-jack@suse.cz Signed-off-by: Jens Axboe --- block/blk-mq-sched.c | 12 +++++++++++- 1 file changed, 11 insertions(+), 1 deletion(-) (limited to 'block/blk-mq-sched.c') diff --git a/block/blk-mq-sched.c b/block/blk-mq-sched.c index 045b6878b8c5..a9182d2f8ad3 100644 --- a/block/blk-mq-sched.c +++ b/block/blk-mq-sched.c @@ -168,9 +168,19 @@ static int __blk_mq_do_dispatch_sched(struct blk_mq_hw_ctx *hctx) * in blk_mq_dispatch_rq_list(). */ list_add_tail(&rq->queuelist, &rq_list); + count++; if (rq->mq_hctx != hctx) multi_hctxs = true; - } while (++count < max_dispatch); + + /* + * If we cannot get tag for the request, stop dequeueing + * requests from the IO scheduler. We are unlikely to be able + * to submit them anyway and it creates false impression for + * scheduling heuristics that the device can take more IO. + */ + if (!blk_mq_get_driver_tag(rq)) + break; + } while (count < max_dispatch); if (!count) { if (run_queue) -- cgit