On Mon, 11 Apr 2011 11:19:58 +0200 Jens Axboe [off-list ref] wrote:

On 2011-04-11 06:50, NeilBrown wrote:

quoted

The only explanation I can come up with is that very occasionally schedule on
2 separate cpus calls blk_flush_plug for the same task.  I don't understand
the scheduler nearly well enough to know if or how that can happen.
However with this patch in place I can write to a RAID1 constantly for half
an hour, and without it, the write rarely lasts for 3 minutes.

Or perhaps if the request_fn blocks, that would be problematic. So the
patch is likely a good idea even for that case.

I'll merge it, changing it to list_splice_init() as I think that would
be more clear.

OK - though I'm not 100% the patch fixes the problem - just that it hides the
symptom for me.
I might try instrumenting the code a bit more and see if I can find exactly
where it is re-entering flush_plug_list - as that seems to be what is
happening.

And yeah - list_split_init is probably better.  I just never remember exactly
what list_split means and have to look it up every time, where as
list_add/list_del are very clear to me.

quoted

From 687b189c02276887dd7d5b87a817da9f67ed3c2c Mon Sep 17 00:00:00 2001
From: NeilBrown <redacted>
Date: Thu, 7 Apr 2011 13:16:59 +1000
Subject: [PATCH] Enhance new plugging support to support general callbacks.

md/raid requires an unplug callback, but as it does not uses
requests the current code cannot provide one.

So allow arbitrary callbacks to be attached to the blk_plug.

Cc: Jens Axboe <redacted>
Signed-off-by: NeilBrown <redacted>
---
 block/blk-core.c       |   13 +++++++++++++
 include/linux/blkdev.h |    7 ++++++-
 2 files changed, 19 insertions(+), 1 deletions(-)

diff --git a/block/blk-core.c b/block/blk-core.c
index 725091d..273d60b 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c

@@ -2644,6 +2644,7 @@ void blk_start_plug(struct blk_plug *plug)
 
 	plug->magic = PLUG_MAGIC;
 	INIT_LIST_HEAD(&plug->list);
+	INIT_LIST_HEAD(&plug->cb_list);
 	plug->should_sort = 0;
 
 	/*

@@ -2717,9 +2718,21 @@ static void flush_plug_list(struct blk_plug *plug)
 	local_irq_restore(flags);
 }
 
+static void flush_plug_callbacks(struct blk_plug *plug)
+{
+	while (!list_empty(&plug->cb_list)) {
+		struct blk_plug_cb *cb = list_first_entry(&plug->cb_list,
+							  struct blk_plug_cb,
+							  list);
+		list_del(&cb->list);
+		cb->callback(cb);
+	}
+}
+
 static void __blk_finish_plug(struct task_struct *tsk, struct blk_plug *plug)
 {
 	flush_plug_list(plug);
+	flush_plug_callbacks(plug);
 
 	if (plug == tsk->plug)
 		tsk->plug = NULL;

diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index 32176cc..3e5e604 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h

@@ -857,8 +857,13 @@ extern void blk_put_queue(struct request_queue *);
 struct blk_plug {
 	unsigned long magic;
 	struct list_head list;
+	struct list_head cb_list;
 	unsigned int should_sort;
 };
+struct blk_plug_cb {
+	struct list_head list;
+	void (*callback)(struct blk_plug_cb *);
+};
 
 extern void blk_start_plug(struct blk_plug *);
 extern void blk_finish_plug(struct blk_plug *);

@@ -876,7 +881,7 @@ static inline bool blk_needs_flush_plug(struct task_struct *tsk)
 {
 	struct blk_plug *plug = tsk->plug;
 
-	return plug && !list_empty(&plug->list);
+	return plug && (!list_empty(&plug->list) || !list_empty(&plug->cb_list));
 }
 
 /*

Maybe I'm missing something, but why do you need those callbacks? If
it's to use plugging yourself, perhaps we can just ensure that those
don't get assigned in the task - so it would be have to used with care.

It's not that I disagree to these callbacks, I just want to ensure I
understand why you need them.

I'm sure one of us is missing something (probably both) but I'm not sure what.

The callback is central.

It is simply to use plugging in md.
Just like blk-core, md will notice that a blk_plug is active and will put
requests aside.  I then need something to call in to md when blk_finish_plug
is called so that put-aside requests can be released.
As md can be built as a module, that call must be a call-back of some sort.
blk-core doesn't need to register blk_plug_flush because that is never in a
module, so it can be called directly.  But the md equivalent could be in a
module, so I need to be able to register a call back.

Does that help? 

Thanks,
NeilBrown