On Tue, Jul 21, 2020 at 12:23:39PM +0530, Kashyap Desai wrote: > > > > > > > > Perf top (shared host tag. IOPS = 230K) > > > > > > > > 13.98% [kernel] [k] sbitmap_any_bit_set > > > > 6.43% [kernel] [k] blk_mq_run_hw_queue > > > > > > blk_mq_run_hw_queue function take more CPU which is called from " > > > scsi_end_request" > > > > The problem could be that nr_hw_queues is increased a lot so that sample > on > > blk_mq_run_hw_queue() can be observed now. > > Yes. That is correct. > > > > > > It looks like " blk_mq_hctx_has_pending" handles only elevator > > > (scheduler) case. If queue has ioscheduler=none, we can skip. I case > > > of scheduler=none, IO will be pushed to hardware queue and it by pass > > software queue. > > > Based on above understanding, I added below patch and I can see > > > performance scale back to expectation. > > > > > > Ming mentioned that - we cannot remove blk_mq_run_hw_queues() from IO > > > completion path otherwise we may see IO hang. So I have just modified > > > completion path assuming it is only required for IO scheduler case. > > > https://www.spinics.net/lists/linux-block/msg55049.html > > > > > > Please review and let me know if this is good or we have to address > > > with proper fix. > > > > > > diff --git a/block/blk-mq.c b/block/blk-mq.c index > > > 1be7ac5a4040..b6a5b41b7fc2 100644 > > > --- a/block/blk-mq.c > > > +++ b/block/blk-mq.c > > > @@ -1559,6 +1559,9 @@ void blk_mq_run_hw_queues(struct > > request_queue > > > *q, bool async) > > > struct blk_mq_hw_ctx *hctx; > > > int i; > > > > > > + if (!q->elevator) > > > + return; > > > + > > > > This way shouldn't be correct, blk_mq_run_hw_queues() is still needed > for > > none because request may not be dispatched successfully by direct issue. > > When block layer attempt posting request to h/w queue directly (for > ioscheduler=none) and if it fails, it is calling > blk_mq_request_bypass_insert(). > blk_mq_request_bypass_insert function will start the h/w queue from > submission context. Do we still have an issue if we skip running hw queue > from completion ? The thing is that we can't guarantee that direct issue or adding request into hctx->dispatch is always done for MQ/none, for example, request still can be added to sw queue from blk_mq_flush_plug_list() when mq plug is applied. Also, I am not sure it is a good idea to add request into hctx->dispatch via blk_mq_request_bypass_insert() in __blk_mq_try_issue_directly() in case of running out of budget, because this way may hurt sequential IO performance. Thanks, Ming
