From mboxrd@z Thu Jan  1 00:00:00 1970
From: Dave Hansen <dave.hansen@intel.com>
Subject: Re: [PATCH] Avoid useless inodes and dentries reclamation
Date: Thu, 29 Aug 2013 11:36:10 -0700
Message-ID: <521F949A.2020908@intel.com>
References: <1377726732.3625.31.camel@schen9-DESK>  <20130829110741.GA23571@dastard> <1377799676.3625.69.camel@schen9-DESK>
Mime-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 7bit
Cc: Dave Chinner <david@fromorbit.com>,
	Alexander Viro <viro@zeniv.linux.org.uk>,
	Jan Kara <jack@suse.cz>, Dave Chinner <dchinner@redhat.com>,
	Andi Kleen <ak@linux.intel.com>,
	Matthew Wilcox <willy@linux.intel.com>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	linux-kernel <linux-kernel@vger.kernel.org>
To: Tim Chen <tim.c.chen@linux.intel.com>
Return-path: <linux-kernel-owner@vger.kernel.org>
In-Reply-To: <1377799676.3625.69.camel@schen9-DESK>
Sender: linux-kernel-owner@vger.kernel.org
List-Id: linux-fsdevel.vger.kernel.org

The new shrinker infrastructure in mmotm looks like it will make this
problem worse.

old code:
shrink_slab()
	for_each_shrinker {
		do_shrinker_shrink(); // one per batch
			prune_super()
				grab_super_passive()
	}
}

Which means we've got at _most_ one grab_super_passive() per batch.  The
new code is something like this:

shrink_slab()
{
	list_for_each_entry(shrinker, &shrinker_list, list) {
                for_each_node_mask(... shrinkctl->nodes_to_scan) {
			shrink_slab_node()
		}
	}
}

shrink_slab_node()
{
        max_pass = shrinker->count_objects(shrinker, shrinkctl);
	// ^^ does grab_super_passive()
	...
	while (total_scan >= batch_size) {
		ret = shrinker->scan_objects(shrinker, shrinkctl);
		// ^^ does grab_super_passive()
	}
}

We've got an extra grab_super_passive()s in the case where we are
actually doing a scan, plus we've got the extra for_each_node_mask()
loop.  That means even more lock acquisitions in the multi-node NUMA
case, which is exactly where we want to get rid of global lock acquisitions.