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Commit 75df7136 authored by Suresh Jayaraman's avatar Suresh Jayaraman Committed by Jens Axboe
Browse files

block: document blk-plug 

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: default avatarSuresh Jayaraman <sjayaraman@suse.de>
Cc: Shaohua Li <shaohua.li@intel.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: default avatarAndrew Morton <akpm@google.com>
Signed-off-by: default avatarJens Axboe <axboe@kernel.dk>
parent 27a84d54

block/blk-core.c

+14 −0
Original line number Diff line number Diff line
@@ -2595,6 +2595,20 @@ EXPORT_SYMBOL(kblockd_schedule_delayed_work); 


#define PLUG_MAGIC	0x91827364



/**

 * blk_start_plug - initialize blk_plug and track it inside the task_struct

 * @plug:	The &struct blk_plug that needs to be initialized

 *

 * Description:

 *   Tracking blk_plug inside the task_struct will help with auto-flushing the

 *   pending I/O should the task end up blocking between blk_start_plug() and

 *   blk_finish_plug(). This is important from a performance perspective, but

 *   also ensures that we don't deadlock. For instance, if the task is blocking

 *   for a memory allocation, memory reclaim could end up wanting to free a

 *   page belonging to that request that is currently residing in our private

 *   plug. By flushing the pending I/O when the process goes to sleep, we avoid

 *   this kind of deadlock.

 */

void blk_start_plug(struct blk_plug *plug)

{

	struct task_struct *tsk = current;

include/linux/blkdev.h

+15 −9
Original line number Diff line number Diff line
@@ -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