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Linux currently has two models for block devices: - The classic request_fn based approach, where drivers use struct request units for IO. The block layer provides various helper functionalities to let drivers share code, things like tag management, timeout handling, queueing, etc. - The "stacked" approach, where a driver squeezes in between the block layer and IO submitter. Since this bypasses the IO stack, driver generally have to manage everything themselves. With drivers being written for new high IOPS devices, the classic request_fn based driver doesn't work well enough. The design dates back to when both SMP and high IOPS was rare. It has problems with scaling to bigger machines, and runs into scaling issues even on smaller machines when you have IOPS in the hundreds of thousands per device. The stacked approach is then most often selected as the model for the driver. But this means that everybody has to re-invent everything, and along with that we get all the problems again that the shared approach solved. This commit introduces blk-mq, block multi queue support. The design is centered around per-cpu queues for queueing IO, which then funnel down into x number of hardware submission queues. We might have a 1:1 mapping between the two, or it might be an N:M mapping. That all depends on what the hardware supports. blk-mq provides various helper functions, which include: - Scalable support for request tagging. Most devices need to be able to uniquely identify a request both in the driver and to the hardware. The tagging uses per-cpu caches for freed tags, to enable cache hot reuse. - Timeout handling without tracking request on a per-device basis. Basically the driver should be able to get a notification, if a request happens to fail. - Optional support for non 1:1 mappings between issue and submission queues. blk-mq can redirect IO completions to the desired location. - Support for per-request payloads. Drivers almost always need to associate a request structure with some driver private command structure. Drivers can tell blk-mq this at init time, and then any request handed to the driver will have the required size of memory associated with it. - Support for merging of IO, and plugging. The stacked model gets neither of these. Even for high IOPS devices, merging sequential IO reduces per-command overhead and thus increases bandwidth. For now, this is provided as a potential 3rd queueing model, with the hope being that, as it matures, it can replace both the classic and stacked model. That would get us back to having just 1 real model for block devices, leaving the stacked approach to dm/md devices (as it was originally intended). Contributions in this patch from the following people: Shaohua Li <shli@fusionio.com> Alexander Gordeev <agordeev@redhat.com> Christoph Hellwig <hch@infradead.org> Mike Christie <michaelc@cs.wisc.edu> Matias Bjorling <m@bjorling.me> Jeff Moyer <jmoyer@redhat.com> Acked-by: Christoph Hellwig <hch@lst.de> Signed-off-by: Jens Axboe <axboe@kernel.dk>
53 lines
1.4 KiB
C
53 lines
1.4 KiB
C
#ifndef INT_BLK_MQ_H
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#define INT_BLK_MQ_H
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struct blk_mq_ctx {
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struct {
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spinlock_t lock;
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struct list_head rq_list;
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} ____cacheline_aligned_in_smp;
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unsigned int cpu;
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unsigned int index_hw;
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unsigned int ipi_redirect;
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/* incremented at dispatch time */
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unsigned long rq_dispatched[2];
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unsigned long rq_merged;
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/* incremented at completion time */
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unsigned long ____cacheline_aligned_in_smp rq_completed[2];
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struct request_queue *queue;
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struct kobject kobj;
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};
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void __blk_mq_end_io(struct request *rq, int error);
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void blk_mq_complete_request(struct request *rq, int error);
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void blk_mq_run_request(struct request *rq, bool run_queue, bool async);
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void blk_mq_run_hw_queue(struct blk_mq_hw_ctx *hctx, bool async);
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void blk_mq_init_flush(struct request_queue *q);
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/*
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* CPU hotplug helpers
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*/
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struct blk_mq_cpu_notifier;
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void blk_mq_init_cpu_notifier(struct blk_mq_cpu_notifier *notifier,
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void (*fn)(void *, unsigned long, unsigned int),
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void *data);
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void blk_mq_register_cpu_notifier(struct blk_mq_cpu_notifier *notifier);
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void blk_mq_unregister_cpu_notifier(struct blk_mq_cpu_notifier *notifier);
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void blk_mq_cpu_init(void);
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DECLARE_PER_CPU(struct llist_head, ipi_lists);
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/*
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* CPU -> queue mappings
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*/
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struct blk_mq_reg;
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extern unsigned int *blk_mq_make_queue_map(struct blk_mq_reg *reg);
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extern int blk_mq_update_queue_map(unsigned int *map, unsigned int nr_queues);
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void blk_mq_add_timer(struct request *rq);
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#endif
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