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scsi_mq_ops里面的commit_rqs：
用来一批IO一起发送时，在最后一个IO时才写doorbell。
使用场景：
block层unplug时候会产生一批IO：
do while把所有plug的IO都下发

void blk_mq_flush_plug_list(struct blk_plug *plug, bool from_schedule) { LIST_HEAD(list); if (list_empty(&plug->mq_list)) return; list_splice_init(&plug->mq_list, &list); if (plug->rq_count > 2 && plug->multiple_queues) list_sort(NULL, &list, plug_rq_cmp); plug->rq_count = 0; do { struct list_head rq_list; struct request *rq, *head_rq = list_entry_rq(list.next); struct list_head *pos = &head_rq->queuelist; /* skip first */ struct blk_mq_hw_ctx *this_hctx = head_rq->mq_hctx; struct blk_mq_ctx *this_ctx = head_rq->mq_ctx; unsigned int depth = 1; list_for_each_continue(pos, &list) { rq = list_entry_rq(pos); BUG_ON(!rq->q); if (rq->mq_hctx != this_hctx || rq->mq_ctx != this_ctx) break; depth++; }list_cut_before(&rq_list, &list, pos); trace_block_unplug(head_rq->q, depth, !from_schedule); blk_mq_sched_insert_requests(this_hctx, this_ctx, &rq_list, from_schedule); } while(!list_empty(&list)); }

为什么要最后一个IO才写doorbell：
适用于写doorbell开销比较大的场合，例如虚拟化硬件（比如virtio-scsi）

if (shost->hostt->commit_rqs) tag_set->ops = &scsi_mq_ops; else tag_set->ops = &scsi_mq_ops_no_commit;

block层调用commit_rqs，场合一：

void blk_mq_try_issue_list_directly(struct blk_mq_hw_ctx *hctx, struct list_head *list) { int queued = 0; int errors = 0; while (!list_empty(list)) { blk_status_t ret; struct request *rq = list_first_entry(list, struct request, queuelist); list_del_init(&rq->queuelist); ret = blk_mq_request_issue_directly(rq, list_empty(list)); if (ret != BLK_STS_OK) { if (ret == BLK_STS_RESOURCE || ret == BLK_STS_DEV_RESOURCE) { blk_mq_request_bypass_insert(rq, false, list_empty(list)); break; } blk_mq_end_request(rq, ret); errors++; } else queued++; }/* * If we didn't flush the entire list, we could have told * the driver there was more coming, but that turned out to * be a lie. */ if ((!list_empty(list) || errors) && hctx->queue->mq_ops->commit_rqs && queued) hctx->queue->mq_ops->commit_rqs(hctx); }

【scsi】参考https://lore.kernel.org/lkml/...