On Mon, Oct 30, 2017 at 08:24:57PM +0000, Bart Van Assche wrote: > On Fri, 2017-10-27 at 13:38 +0800, Ming Lei wrote: > > On Fri, Oct 27, 2017 at 04:53:18AM +0000, Bart Van Assche wrote: > > > On Fri, 2017-10-27 at 12:43 +0800, Ming Lei wrote: > > > > The 1st patch removes the RESTART for TAG-SHARED because SCSI handles it > > > > by itself, and not necessary to waste CPU to do the expensive RESTART. > > > > And Roman Pen reported that this RESTART cuts half of IOPS in his case. > > > > > > > > The 2nd patch removes the RESTART when .get_budget returns BLK_STS_RESOURCE, > > > > and this RESTART is handled by SCSI's RESTART(scsi_end_request()) too. > > > > > > There are more block drivers than the SCSI core that share tags. If the > > > > Could you share us what the other in-tree driver which share tags is? > > I think the following in-tree drivers support shared tags (in alphabetical > order): > * null_blk. See also the shared_tags kernel module parameter. > * nvme. See also nvme_alloc_ns(). For both null_blk and nvme, we don't need to deal with cross-queue RESTART, because BLK_MQ_S_TAG_WAITING has handled it already. > * scsi-mq. > > Please note that the queue_rq() function of *all* these drivers can return > BLK_STS_RESOURCE. Yes, but this patch doesn't remove the RESTART for the current hctx, and only removed the part of handling cross-queue RESTART. > > > Actually SCSI's RESTART is very thin, and it is just the per-host starved_list. > > Do you see any reason that would prevent to move that mechanism from scsi-mq > into the blk-mq core? I don't see it is required from SCSI's view since this way has been run for SCSI for long time, and we don't have this requirement for other in-tree drivers meantime. Also as I mentioned before, driver has more knowledge than generic block layer for making it more efficiently. Thanks, Ming
