linux-kernel - Re: [PATCH] Avoid useless inodes and dentries reclamation

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Date:	Fri, 30 Aug 2013 09:21:34 -0700
From:	Tim Chen <tim.c.chen@...ux.intel.com>
To:	Dave Chinner <david@...morbit.com>
Cc:	Alexander Viro <viro@...iv.linux.org.uk>, Jan Kara <jack@...e.cz>,
	Dave Chinner <dchinner@...hat.com>,
	Dave Hansen <dave.hansen@...el.com>,
	Andi Kleen <ak@...ux.intel.com>,
	Matthew Wilcox <willy@...ux.intel.com>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] Avoid useless inodes and dentries reclamation

On Fri, 2013-08-30 at 11:40 +1000, Dave Chinner wrote:

> 
> The new shrinker infrastructure has a ->count_objects() callout to
> specifically return the number of objects in the cache.
> shrink_slab_node() can check that return value against the "minimum
> call count" and determine whether it needs to call ->scan_objects()
> at all.
> 
> Actually, the shrinker already behaves like this with the batch_size
> variable - the shrinker has to be asking for more items to be
> scanned than the batch size. That means the problem is that counting
> callouts are causing the problem, not the scanning callouts.
> 
> With the new code in the mmotm tree, for counting purposes we
> probably don't need to grab a passive superblock reference at all -
> the superblock and LRUs are guaranteed to be valid if the shrinker
> is currently running, but we don't really care if the superblock is
> being shutdown and the values that come back are invalid because the
> ->scan_objects() callout will fail to grab the superblock to do
> anything with the calculated values.

If that's the case, then we should remove grab_super_passive
from the super_cache_count code.  That should remove the bottleneck
in reclamation.

Thanks for your detailed explanation.

Tim

Signed-off-by: Tim Chen <tim.c.chen@...ux.intel.com>
---
diff --git a/fs/super.c b/fs/super.c
index 73d0952..4df1fab 100644
--- a/fs/super.c
+++ b/fs/super.c
@@ -112,9 +112,6 @@ static unsigned long super_cache_count(struct shrinker *shrink,
 
 	sb = container_of(shrink, struct super_block, s_shrink);
 
-	if (!grab_super_passive(sb))
-		return 0;
-
 	if (sb->s_op && sb->s_op->nr_cached_objects)
 		total_objects = sb->s_op->nr_cached_objects(sb,
 						 sc->nid);




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