The patch titled
block: document blk-plug
has been added to the -mm tree. Its filename is
block-document-blk-plug.patch
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Subject: block: document blk-plug
From: Suresh Jayaraman <sjayaraman@xxxxxxx>
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.
Signed-off-by: Suresh Jayaraman <sjayaraman@xxxxxxx>
Cc: Jens Axboe <axboe@xxxxxxxxx>
Cc: Shaohua Li <shaohua.li@xxxxxxxxx>
Cc: Jonathan Corbet <corbet@xxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
block/blk-core.c | 14 ++++++++++++++
include/linux/blkdev.h | 16 +++++++++++-----
2 files changed, 25 insertions(+), 5 deletions(-)
diff -puN block/blk-core.c~block-document-blk-plug block/blk-core.c
--- a/block/blk-core.c~block-document-blk-plug
+++ a/block/blk-core.c
@@ -2626,6 +2626,20 @@ EXPORT_SYMBOL(kblockd_schedule_delayed_w
#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 deadlocks.
+ */
void blk_start_plug(struct blk_plug *plug)
{
struct task_struct *tsk = current;
diff -puN include/linux/blkdev.h~block-document-blk-plug include/linux/blkdev.h
--- a/include/linux/blkdev.h~block-document-blk-plug
+++ a/include/linux/blkdev.h
@@ -863,17 +863,23 @@ struct request_queue *blk_alloc_queue_no
extern void blk_put_queue(struct request_queue *);
/*
+ * blk_plug allows to build up 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 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.
+ *
* 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.
*/
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; /* request count to avoid list getting too big */
};
#define BLK_MAX_REQUEST_COUNT 16
_
Patches currently in -mm which might be from sjayaraman@xxxxxxx are
block-document-blk-plug.patch
block-document-blk-plug-fix.patch
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