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block: document blk-plug

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Thus spake Andrew Morton:

"And I have the usual maintainability whine.  If someone comes up to
vmscan.c and sees it calling blk_start_plug(), how are they supposed to
work out why that call is there?  They go look at the blk_start_plug()
definition and it is undocumented.  I think we can do better than this?"

Adapted from the LWN article - http://lwn.net/Articles/438256/ by Jens
Axboe and from an earlier attempt by Shaohua Li to document blk-plug.

[akpm@linux-foundation.org: grammatical and spelling tweaks]
Signed-off-by: Suresh Jayaraman <sjayaraman@suse.de>
Cc: Shaohua Li <shaohua.li@intel.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Andrew Morton <akpm@google.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
This commit is contained in:
Suresh Jayaraman 2011-09-21 10:00:16 +02:00 • committed by Jens Axboe
parent 27a84d54c0
commit 75df713627
2 changed files with 29 additions and 9 deletions
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24
include/linux/blkdev.h
View file

@ -860,17 +860,23 @@ struct request_queue *blk_alloc_queue_node(gfp_t, int);
extern void blk_put_queue(struct request_queue *);
/*
* Note: Code in between changing the blk_plug list/cb_list or element of such
* lists is preemptable, but such code can't do sleep (or be very careful),
* otherwise data is corrupted. For details, please check schedule() where
* blk_schedule_flush_plug() is called.
* blk_plug permits building a queue of related requests by holding the I/O
* fragments for a short period. This allows merging of sequential requests
* into single larger request. As the requests are moved from a per-task list to
* the device's request_queue in a batch, this results in improved scalability
* as the lock contention for request_queue lock is reduced.
*
* It is ok not to disable preemption when adding the request to the plug list
* or when attempting a merge, because blk_schedule_flush_list() will only flush
* the plug list when the task sleeps by itself. For details, please see
* schedule() where blk_schedule_flush_plug() is called.
*/
struct blk_plug {
unsigned long magic;
struct list_head list;
struct list_head cb_list;
unsigned int should_sort;
unsigned int count;
unsigned long magic; /* detect uninitialized use-cases */
struct list_head list; /* requests */
struct list_head cb_list; /* md requires an unplug callback */
unsigned int should_sort; /* list to be sorted before flushing? */
unsigned int count; /* number of queued requests */
};
#define BLK_MAX_REQUEST_COUNT 16