linux-kernel - Re: [PATCH 05/10] block: remove per-queue plugging

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Message-ID: <4DABDC60.2090009@fusionio.com>
Date:	Mon, 18 Apr 2011 08:38:24 +0200
From:	Jens Axboe <jaxboe@...ionio.com>
To:	NeilBrown <neilb@...e.de>
CC:	Mike Snitzer <snitzer@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"hch@...radead.org" <hch@...radead.org>,
	"dm-devel@...hat.com" <dm-devel@...hat.com>,
	"linux-raid@...r.kernel.org" <linux-raid@...r.kernel.org>
Subject: Re: [PATCH 05/10] block: remove per-queue plugging

On 2011-04-18 00:19, NeilBrown wrote:
> On Mon, 11 Apr 2011 14:11:58 +0200 Jens Axboe <jaxboe@...ionio.com> wrote:
> 
>>> Yes.  But I need to know when to release the requests that I have stored.
>>> I need to know when ->write_pages or ->read_pages or whatever has finished
>>> submitting a pile of pages so that I can start processing the request that I
>>> have put aside.  So I need a callback from blk_finish_plug.
>>
>> OK fair enough, I'll add your callback patch.
>>
> 
> But you didn't did you?  You added a completely different patch which is
> completely pointless.
> If you don't like my patch I would really prefer you said so rather than
> silently replace it with something completely different (and broken).

First of all, you were CC'ed on all that discussion, yet didn't speak up
until now. This was last week. Secondly, please change your tone.

> I'll try to explain again.
> 
> md does not use __make_request.  At all.
> md does not use 'struct request'.  At all.
> 
> The 'list' in 'struct blk_plug' is a list of 'struct request'.

I'm well aware of how these facts, but thanks for bringing it up.

> Therefore md cannot put anything useful on the list in 'struct blk_plug'.
> 
> So when blk_flush_plug_list calls queue_unplugged() on a queue that belonged
> to a request found on the blk_plug list, that queue cannot possibly ever be
> for an 'md' device (because no 'struct request' ever belongs to an md device,
> because md doesn't not use 'struct request').
> 
> So your patch (commit f75664570d8b) doesn't help MD at all.
> 
> For md, I need to attach something to blk_plug which somehow identifies an md
> device, so that blk_finish_plug can get to that device and let it unplug.
> The most sensible thing to have is a completely generic callback.  That way
> different block devices (which choose not to use __make_request) can attach
> different sorts of things to blk_plug.
> 
> So can we please have my original patch applied? (Revised version using
> list_splice_init included below).
> 
> Or if not, a clear explanation of why not?

So correct me if I'm wrong here, but the _only_ real difference between
this patch and the current code in the tree, is the checking of the
callback list indicating a need to flush the callbacks. And that's
definitely an oversight. It should be functionally equivelant if md
would just flag this need to get a callback, eg instead of queueing a
callback on the list, just set plug->need_unplug from md instead of
queuing a callback and have blk_needs_flush_plug() do:

        return plug && (!list_empty(&plug->list) || plug->need_unplug);

instead. Something like the below, completely untested.


diff --git a/block/blk-core.c b/block/blk-core.c
index 78b7b0c..e1f5635 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -1305,12 +1305,12 @@ get_rq:
 		 */
 		if (list_empty(&plug->list))
 			trace_block_plug(q);
-		else if (!plug->should_sort) {
+		else if (!(plug->flags & BLK_PLUG_F_SORT)) {
 			struct request *__rq;
 
 			__rq = list_entry_rq(plug->list.prev);
 			if (__rq->q != q)
-				plug->should_sort = 1;
+				plug->flags |= BLK_PLUG_F_SORT;
 		}
 		/*
 		 * Debug flag, kill later
@@ -2638,7 +2638,7 @@ void blk_start_plug(struct blk_plug *plug)
 
 	plug->magic = PLUG_MAGIC;
 	INIT_LIST_HEAD(&plug->list);
-	plug->should_sort = 0;
+	plug->flags = 0;
 
 	/*
 	 * If this is a nested plug, don't actually assign it. It will be
@@ -2693,9 +2693,9 @@ void blk_flush_plug_list(struct blk_plug *plug, bool from_schedule)
 
 	list_splice_init(&plug->list, &list);
 
-	if (plug->should_sort) {
+	if (plug->flags & BLK_PLUG_F_SORT) {
 		list_sort(NULL, &list, plug_rq_cmp);
-		plug->should_sort = 0;
+		plug->flags &= ~BLK_PLUG_F_SORT;
 	}
 
 	q = NULL;
diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index ec0357d..1a0b76b 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -860,7 +860,12 @@ extern void blk_put_queue(struct request_queue *);
 struct blk_plug {
 	unsigned long magic;
 	struct list_head list;
-	unsigned int should_sort;
+	unsigned int flags;
+};
+
+enum {
+	BLK_PLUG_F_SORT		= 1,
+	BLK_PLUG_F_NEED_UNPLUG	= 2,
 };
 
 extern void blk_start_plug(struct blk_plug *);
@@ -887,7 +892,8 @@ 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) ||
+			(plug->flags & BLK_PLUG_F_NEED_UNPLUG));
 }
 
 /*

-- 
Jens Axboe

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