[fullquote removed, please follow proper mail etiquette] On Tue, Feb 19, 2019 at 08:56:28AM -0800, Bart Van Assche wrote: > regression in the SCSI sd driver due to the switch from the legacy block > layer to scsi-mq. The above patch introduces two atomic operations in the > hot path and hence would introduce a performance regression. I think this > can be avoided by making sure that sd_uninit_command() gets called before > the request tag is freed. What changes would be required to make the block > layer core call sd_uninit_command() before the request tag is freed? Would > introducing prep_rq_fn and unprep_rq_fn callbacks in struct blk_mq_ops and > making sure that the SCSI core sets these callback function pointers > appropriately be sufficient? Would such a change allow to simplify the NVMe > initiator driver? Are there any alternatives to this approach that are more > elegant? Additional indirect calls in the I/O fast path is something I'd rather avoid. But I don't fully understand the problem yet - where do we release a disk reference from blk_update_request? And why can't we move that release to __blk_mq_end_request? > > Thanks, > > Bart. ---end quoted text---
