linux-kernel - Re: [PATCH] block: strip out locking optimization in put_io

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Date:	Tue, 7 Feb 2012 09:17:15 -0800
From:	Tejun Heo <tj@...nel.org>
To:	Shaohua Li <vivek.goyal2008@...il.com>
Cc:	Jens Axboe <axboe@...nel.dk>, Vivek Goyal <vgoyal@...hat.com>,
	lkml <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Knut Petersen <Knut_Petersen@...nline.de>, mroos@...ux.ee
Subject: Re: [PATCH] block: strip out locking optimization in
 put_io_context()

On Tue, Feb 07, 2012 at 08:47:35AM -0800, Tejun Heo wrote:
> Yeah, I was about to ask Shaohua to test the version w/o optimization.
> With heavily loaded request_queue, trylock failure could be frequent,
> which I wasn't testing.
> 
> Shaohua, can you please test the version w/o optimization?  Also, can
> you please give a bit more details on the setup?  Are there multiple
> swap devices?  Is it SSD or rotating disk?

Can you please also apply the following patch on top of
block/for-linus and see how it behaves?

Thanks.
---
 block/blk-ioc.c |   54 +++++++++++++++++-------------------------------------
 1 file changed, 17 insertions(+), 37 deletions(-)

Index: work/block/blk-ioc.c
===================================================================
--- work.orig/block/blk-ioc.c
+++ work/block/blk-ioc.c
@@ -79,52 +79,32 @@ static void ioc_release_fn(struct work_s
 {
 	struct io_context *ioc = container_of(work, struct io_context,
 					      release_work);
-	struct request_queue *last_q = NULL;
+	unsigned long flags;
 
-	spin_lock_irq(&ioc->lock);
+	/*
+	 * Exiting icq may call into put_io_context() through elevator
+	 * which will trigger lockdep warning.  The ioc's are guaranteed to
+	 * be different, use a different locking subclass here.  Using
+	 * irqsave variant as there's no spin_lock_irq_nested().
+	 */
+	spin_lock_irqsave_nested(&ioc->lock, flags, 1);
 
 	while (!hlist_empty(&ioc->icq_list)) {
 		struct io_cq *icq = hlist_entry(ioc->icq_list.first,
 						struct io_cq, ioc_node);
-		struct request_queue *this_q = icq->q;
+		struct request_queue *q = icq->q;
 
-		if (this_q != last_q) {
-			/*
-			 * Need to switch to @this_q.  Once we release
-			 * @ioc->lock, it can go away along with @cic.
-			 * Hold on to it.
-			 */
-			__blk_get_queue(this_q);
-
-			/*
-			 * blk_put_queue() might sleep thanks to kobject
-			 * idiocy.  Always release both locks, put and
-			 * restart.
-			 */
-			if (last_q) {
-				spin_unlock(last_q->queue_lock);
-				spin_unlock_irq(&ioc->lock);
-				blk_put_queue(last_q);
-			} else {
-				spin_unlock_irq(&ioc->lock);
-			}
-
-			last_q = this_q;
-			spin_lock_irq(this_q->queue_lock);
-			spin_lock(&ioc->lock);
-			continue;
+		if (spin_trylock(q->queue_lock)) {
+			ioc_exit_icq(icq);
+			spin_unlock(q->queue_lock);
+		} else {
+			spin_unlock_irqrestore(&ioc->lock, flags);
+			cpu_relax();
+			spin_lock_irqsave_nested(&ioc->lock, flags, 1);
 		}
-		ioc_exit_icq(icq);
-	}
-
-	if (last_q) {
-		spin_unlock(last_q->queue_lock);
-		spin_unlock_irq(&ioc->lock);
-		blk_put_queue(last_q);
-	} else {
-		spin_unlock_irq(&ioc->lock);
 	}
 
+	spin_unlock_irqrestore(&ioc->lock, flags);
 	kmem_cache_free(iocontext_cachep, ioc);
 }
 
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