blk-mq: new multi-queue block IO queueing mechanism
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>
2013-10-24 15:20:05 +07:00
|
|
|
#ifndef INT_BLK_MQ_H
|
|
|
|
|
#define INT_BLK_MQ_H
|
|
|
|
|
|
2016-11-08 11:32:37 +07:00
|
|
|
#include "blk-stat.h"
|
|
|
|
|
|
2014-04-16 03:14:00 +07:00
|
|
|
struct blk_mq_tag_set;
|
|
|
|
|
|
blk-mq: new multi-queue block IO queueing mechanism
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>
2013-10-24 15:20:05 +07:00
|
|
|
struct blk_mq_ctx {
|
|
|
|
|
struct {
|
|
|
|
|
spinlock_t lock;
|
|
|
|
|
struct list_head rq_list;
|
|
|
|
|
} ____cacheline_aligned_in_smp;
|
|
|
|
|
|
|
|
|
|
unsigned int cpu;
|
|
|
|
|
unsigned int index_hw;
|
|
|
|
|
|
|
|
|
|
/* incremented at dispatch time */
|
|
|
|
|
unsigned long rq_dispatched[2];
|
|
|
|
|
unsigned long rq_merged;
|
|
|
|
|
|
|
|
|
|
/* incremented at completion time */
|
|
|
|
|
unsigned long ____cacheline_aligned_in_smp rq_completed[2];
|
|
|
|
|
|
|
|
|
|
struct request_queue *queue;
|
|
|
|
|
struct kobject kobj;
|
2014-05-09 22:36:49 +07:00
|
|
|
} ____cacheline_aligned_in_smp;
|
blk-mq: new multi-queue block IO queueing mechanism
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>
2013-10-24 15:20:05 +07:00
|
|
|
|
|
|
|
|
void blk_mq_run_hw_queue(struct blk_mq_hw_ctx *hctx, bool async);
|
2014-07-01 23:31:13 +07:00
|
|
|
void blk_mq_freeze_queue(struct request_queue *q);
|
2013-12-26 20:31:38 +07:00
|
|
|
void blk_mq_free_queue(struct request_queue *q);
|
2014-05-21 00:49:02 +07:00
|
|
|
int blk_mq_update_nr_requests(struct request_queue *q, unsigned int nr);
|
2014-12-23 04:04:42 +07:00
|
|
|
void blk_mq_wake_waiters(struct request_queue *q);
|
2017-10-14 16:22:29 +07:00
|
|
|
bool blk_mq_dispatch_rq_list(struct request_queue *, struct list_head *, bool);
|
2016-12-15 04:34:47 +07:00
|
|
|
void blk_mq_flush_busy_ctxs(struct blk_mq_hw_ctx *hctx, struct list_head *list);
|
2017-01-27 04:42:34 +07:00
|
|
|
bool blk_mq_hctx_has_pending(struct blk_mq_hw_ctx *hctx);
|
2017-01-27 15:00:47 +07:00
|
|
|
bool blk_mq_get_driver_tag(struct request *rq, struct blk_mq_hw_ctx **hctx,
|
|
|
|
|
bool wait);
|
blk-mq-sched: improve dispatching from sw queue
SCSI devices use host-wide tagset, and the shared driver tag space is
often quite big. However, there is also a queue depth for each lun(
.cmd_per_lun), which is often small, for example, on both lpfc and
qla2xxx, .cmd_per_lun is just 3.
So lots of requests may stay in sw queue, and we always flush all
belonging to same hw queue and dispatch them all to driver.
Unfortunately it is easy to cause queue busy because of the small
.cmd_per_lun. Once these requests are flushed out, they have to stay in
hctx->dispatch, and no bio merge can happen on these requests, and
sequential IO performance is harmed.
This patch introduces blk_mq_dequeue_from_ctx for dequeuing a request
from a sw queue, so that we can dispatch them in scheduler's way. We can
then avoid dequeueing too many requests from sw queue, since we don't
flush ->dispatch completely.
This patch improves dispatching from sw queue by using the .get_budget
and .put_budget callbacks.
Reviewed-by: Omar Sandoval <osandov@fb.com>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
2017-10-14 16:22:30 +07:00
|
|
|
struct request *blk_mq_dequeue_from_ctx(struct blk_mq_hw_ctx *hctx,
|
|
|
|
|
struct blk_mq_ctx *start);
|
2016-12-15 04:34:47 +07:00
|
|
|
|
|
|
|
|
/*
|
|
|
|
|
* Internal helpers for allocating/freeing the request map
|
|
|
|
|
*/
|
2017-01-12 04:29:56 +07:00
|
|
|
void blk_mq_free_rqs(struct blk_mq_tag_set *set, struct blk_mq_tags *tags,
|
|
|
|
|
unsigned int hctx_idx);
|
|
|
|
|
void blk_mq_free_rq_map(struct blk_mq_tags *tags);
|
|
|
|
|
struct blk_mq_tags *blk_mq_alloc_rq_map(struct blk_mq_tag_set *set,
|
|
|
|
|
unsigned int hctx_idx,
|
|
|
|
|
unsigned int nr_tags,
|
|
|
|
|
unsigned int reserved_tags);
|
|
|
|
|
int blk_mq_alloc_rqs(struct blk_mq_tag_set *set, struct blk_mq_tags *tags,
|
|
|
|
|
unsigned int hctx_idx, unsigned int depth);
|
2016-12-15 04:34:47 +07:00
|
|
|
|
|
|
|
|
/*
|
|
|
|
|
* Internal helpers for request insertion into sw queues
|
|
|
|
|
*/
|
|
|
|
|
void __blk_mq_insert_request(struct blk_mq_hw_ctx *hctx, struct request *rq,
|
|
|
|
|
bool at_head);
|
2017-09-12 05:43:57 +07:00
|
|
|
void blk_mq_request_bypass_insert(struct request *rq);
|
2017-01-17 20:03:22 +07:00
|
|
|
void blk_mq_insert_requests(struct blk_mq_hw_ctx *hctx, struct blk_mq_ctx *ctx,
|
|
|
|
|
struct list_head *list);
|
blk-mq: new multi-queue block IO queueing mechanism
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>
2013-10-24 15:20:05 +07:00
|
|
|
|
|
|
|
|
/*
|
|
|
|
|
* CPU -> queue mappings
|
|
|
|
|
*/
|
2014-05-28 01:06:53 +07:00
|
|
|
extern int blk_mq_hw_queue_to_node(unsigned int *map, unsigned int);
|
blk-mq: new multi-queue block IO queueing mechanism
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>
2013-10-24 15:20:05 +07:00
|
|
|
|
2016-09-14 21:18:54 +07:00
|
|
|
static inline struct blk_mq_hw_ctx *blk_mq_map_queue(struct request_queue *q,
|
|
|
|
|
int cpu)
|
|
|
|
|
{
|
|
|
|
|
return q->queue_hw_ctx[q->mq_map[cpu]];
|
|
|
|
|
}
|
|
|
|
|
|
2014-05-30 21:25:36 +07:00
|
|
|
/*
|
|
|
|
|
* sysfs helpers
|
|
|
|
|
*/
|
2017-02-22 17:13:59 +07:00
|
|
|
extern void blk_mq_sysfs_init(struct request_queue *q);
|
2017-02-22 17:14:00 +07:00
|
|
|
extern void blk_mq_sysfs_deinit(struct request_queue *q);
|
2017-04-27 03:47:48 +07:00
|
|
|
extern int __blk_mq_register_dev(struct device *dev, struct request_queue *q);
|
2014-05-30 21:25:36 +07:00
|
|
|
extern int blk_mq_sysfs_register(struct request_queue *q);
|
|
|
|
|
extern void blk_mq_sysfs_unregister(struct request_queue *q);
|
2015-12-18 07:08:14 +07:00
|
|
|
extern void blk_mq_hctx_kobj_init(struct blk_mq_hw_ctx *hctx);
|
2014-05-30 21:25:36 +07:00
|
|
|
|
2014-09-22 23:21:48 +07:00
|
|
|
extern void blk_mq_rq_timed_out(struct request *req, bool reserved);
|
|
|
|
|
|
2015-01-29 19:17:27 +07:00
|
|
|
void blk_mq_release(struct request_queue *q);
|
|
|
|
|
|
2014-05-31 23:43:36 +07:00
|
|
|
static inline struct blk_mq_ctx *__blk_mq_get_ctx(struct request_queue *q,
|
|
|
|
|
unsigned int cpu)
|
|
|
|
|
{
|
|
|
|
|
return per_cpu_ptr(q->queue_ctx, cpu);
|
|
|
|
|
}
|
|
|
|
|
|
|
|
|
|
/*
|
|
|
|
|
* This assumes per-cpu software queueing queues. They could be per-node
|
|
|
|
|
* as well, for instance. For now this is hardcoded as-is. Note that we don't
|
|
|
|
|
* care about preemption, since we know the ctx's are persistent. This does
|
|
|
|
|
* mean that we can't rely on ctx always matching the currently running CPU.
|
|
|
|
|
*/
|
|
|
|
|
static inline struct blk_mq_ctx *blk_mq_get_ctx(struct request_queue *q)
|
|
|
|
|
{
|
|
|
|
|
return __blk_mq_get_ctx(q, get_cpu());
|
|
|
|
|
}
|
|
|
|
|
|
|
|
|
|
static inline void blk_mq_put_ctx(struct blk_mq_ctx *ctx)
|
|
|
|
|
{
|
|
|
|
|
put_cpu();
|
|
|
|
|
}
|
|
|
|
|
|
2014-05-31 23:43:37 +07:00
|
|
|
struct blk_mq_alloc_data {
|
|
|
|
|
/* input parameter */
|
|
|
|
|
struct request_queue *q;
|
2015-11-26 15:13:05 +07:00
|
|
|
unsigned int flags;
|
2017-04-14 14:59:59 +07:00
|
|
|
unsigned int shallow_depth;
|
2014-05-31 23:43:37 +07:00
|
|
|
|
|
|
|
|
/* input & output parameter */
|
|
|
|
|
struct blk_mq_ctx *ctx;
|
|
|
|
|
struct blk_mq_hw_ctx *hctx;
|
|
|
|
|
};
|
|
|
|
|
|
2017-01-13 22:09:05 +07:00
|
|
|
static inline struct blk_mq_tags *blk_mq_tags_from_data(struct blk_mq_alloc_data *data)
|
|
|
|
|
{
|
2017-01-17 20:03:22 +07:00
|
|
|
if (data->flags & BLK_MQ_REQ_INTERNAL)
|
|
|
|
|
return data->hctx->sched_tags;
|
|
|
|
|
|
2017-01-13 22:09:05 +07:00
|
|
|
return data->hctx->tags;
|
|
|
|
|
}
|
|
|
|
|
|
2016-10-29 07:19:15 +07:00
|
|
|
static inline bool blk_mq_hctx_stopped(struct blk_mq_hw_ctx *hctx)
|
|
|
|
|
{
|
|
|
|
|
return test_bit(BLK_MQ_S_STOPPED, &hctx->state);
|
|
|
|
|
}
|
|
|
|
|
|
2014-12-03 18:38:04 +07:00
|
|
|
static inline bool blk_mq_hw_queue_mapped(struct blk_mq_hw_ctx *hctx)
|
|
|
|
|
{
|
|
|
|
|
return hctx->nr_ctx && hctx->tags;
|
|
|
|
|
}
|
|
|
|
|
|
2017-08-09 06:51:45 +07:00
|
|
|
void blk_mq_in_flight(struct request_queue *q, struct hd_struct *part,
|
|
|
|
|
unsigned int inflight[2]);
|
|
|
|
|
|
2017-10-14 16:22:29 +07:00
|
|
|
static inline void blk_mq_put_dispatch_budget(struct blk_mq_hw_ctx *hctx)
|
|
|
|
|
{
|
|
|
|
|
struct request_queue *q = hctx->queue;
|
|
|
|
|
|
|
|
|
|
if (q->mq_ops->put_budget)
|
|
|
|
|
q->mq_ops->put_budget(hctx);
|
|
|
|
|
}
|
|
|
|
|
|
|
|
|
|
static inline blk_status_t blk_mq_get_dispatch_budget(
|
|
|
|
|
struct blk_mq_hw_ctx *hctx)
|
|
|
|
|
{
|
|
|
|
|
struct request_queue *q = hctx->queue;
|
|
|
|
|
|
|
|
|
|
if (q->mq_ops->get_budget)
|
|
|
|
|
return q->mq_ops->get_budget(hctx);
|
|
|
|
|
return BLK_STS_OK;
|
|
|
|
|
}
|
|
|
|
|
|
blk-mq: new multi-queue block IO queueing mechanism
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>
2013-10-24 15:20:05 +07:00
|
|
|
#endif
|