On Wed, Jul 22, 2020 at 10:32:41AM +0100, John Garry wrote: > > > > > > > > > > > > > > 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 > > Could the logic of blk_mq_run_hw_queue() -> blk_mq_hctx_has_pending() -> > sbitmap_any_bit_set(&hctx->ctx_map) be optimised for megaraid scenario? > > As I see, since megaraid will have 1:1 mapping of CPU to hw queue, will > there only ever possibly a single bit set in ctx_map? If so, it seems a > waste to always check every sbitmap map. But adding logic for this may > negate any possible gains. It really depends on min and max cpu id in the map, then sbitmap depth can be reduced to (max - min + 1). I'd suggest to double check that cost of sbitmap_any_bit_set() really matters. > > > > > > 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. > > > I see even blk_mq_sched_insert_requests() from blk_mq_flush_plug_list make > > > sure it run the h/w queue. If all the submission path which deals with s/w > > > queue make sure they run h/w queue, can't we remove blk_mq_run_hw_queues() > > > from scsi_end_request ? > > No, one purpose of blk_mq_run_hw_queues() is for rerun queue in case that > > dispatch budget is running out of in submission path, and sdev->device_busy is > > shared by all hw queues on this scsi device. > > > > I posted one patch for avoiding it in scsi_end_request() before, looks it > > never lands upstream: > > > > I saw that you actually posted the v3: > https://lore.kernel.org/linux-scsi/BL0PR2101MB11230C5F70151037B23C0C35CE2D0@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/ > And it no longer applies, due to the changes in scsi_mq_get_budget(), I > think, which look non-trivial. Any chance to repost? OK, will post V4. thanks, Ming
