This series implements support for ZBC disks used through the scsi-mq I/O path. The current scsi level support of ZBC disks guarantees write request ordering using a per-zone write lock which prevents issuing simultaneously multiple write commands to a zone, doing so avoid reordering of sequential writes to sequential zones. This method is however ineffective when scsi-mq is used with zoned block devices. This is due to the different execution model of blk-mq which passes a request to the scsi layer for dispatching after the request has been removed from the I/O scheduler queue. That is, when the scsi layer tries to lock the target zone of the request, the request may already be out of order and zone write locking fails to prevent that. Various approaches have been tried to solve this problem. All of them had the serious disadvantage of cluttering blk-mq code with zoned block device specific conditions and processing. As such extensive changes can only turn into a maintenance nightmares, a radically different solution is proposed here. This series proposes implementing scsi-mq support for zoned block devices in the form of a new "zoned" I/O scheduler based. The zoned I/O scheduler is based on mq-deadline, with most of the code reused with no modification other than name changes. Zoned only differs from mq-deadline from the addition of a per zone write locking mechanism similar to that implemented in sd_zbc.c. The zoned scheduler zone write locking mechanism is used for the exact same purpose as the one in the scsi disk driver, that is, to limit writes per zone to at most one request to avoid reordering. The locking context however changes from that of scsi-mq and is moved to the dispatch_request method of the scheduler. Within this context, under a spin lock guaranteeing atomicity against other dispatch contexts, target zones of write requests can be locked before write requests removal from the scheduler. In effect, this results in the same behavior as the legacy scsi path. Sequential write ordering is preserved. The zoned scheduler is also optimized to not lock conventional zones. To do so, the new data structure blk_zoned is added to the request queue structure so that the low level scsi code can pass information such as number of zones and zone types to the block layer scheduler. This avoids difficulties in accessing this information from the I/O scheduler initialization method (init_queue() method) context. The series patches are as follows: - The first 2 patches fix exported symbols declaration to allow compiling external I/O schedulers as modules. No functional changes are introduced. - Patch 3 introduces the new blk_zoned data structure added to request queues - Pathes 4 to 7 reorganize and cleanup the scsi ZBC support - Path 8 is a bug fix - Path 9 implements the initialization of the blk_zoned structure for zoned block devices - Patch 10 is an optimization for the legacy scsi path which avoids zone locking if a write request targets a conventional zone. - Path 11 disables zone write locking for disks accessed through scsi-mq. - Patch 12 introduces the new zoned blk-mq I/O scheduler. Comments are as always very much appreciated. Changes from v2: * Introduced blk_zoned structure * Moved I/O scheduler from drivers/scsi to block Changes from v1: * Addressed Bart's comments for the blk-mq patches (declarations files) * Split (former) patch 4 into multiple patches to facilitate review * Fixed scsi disk lookup from io scheduler by introducing scsi_disk_from_queue() Damien Le Moal (12): block: Fix declaration of blk-mq debugfs functions block: Fix declaration of blk-mq scheduler functions block: Add zoned block device information to request queue scsi: sd_zbc: Move ZBC declarations to scsi_proto.h scsi: sd_zbc: Fix comments and indentation scsi: sd_zbc: Rearrange code scsi: sd_zbc: Use well defined macros scsi: sd_zbc: Fix sd_zbc_read_zoned_characteristics() scsi: sd_zbc: Initialize device queue zoned structure scsi: sd_zbc: Limit zone write locking to sequential zones scsi: sd_zbc: Disable zone write locking with scsi-mq block: Introduce zoned I/O scheduler Documentation/block/zoned-iosched.txt | 48 ++ block/Kconfig.iosched | 12 + block/Makefile | 1 + block/blk-mq-debugfs.h | 14 +- block/blk-mq-sched.h | 11 +- block/zoned-iosched.c | 925 ++++++++++++++++++++++++++++++++++ drivers/scsi/scsi_lib.c | 5 +- drivers/scsi/sd_zbc.c | 267 +++++++--- include/linux/blk-mq-debugfs.h | 23 + include/linux/blk-mq-sched.h | 14 + include/linux/blkdev.h | 24 + include/scsi/scsi_proto.h | 45 +- 12 files changed, 1283 insertions(+), 106 deletions(-) create mode 100644 Documentation/block/zoned-iosched.txt create mode 100644 block/zoned-iosched.c create mode 100644 include/linux/blk-mq-debugfs.h create mode 100644 include/linux/blk-mq-sched.h -- 2.13.5
