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From: Roman Gushchin <guro@fb.com>
To: "Tobin C. Harding" <me@tobin.cc>
Cc: "Tobin C. Harding" <tobin@kernel.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Matthew Wilcox <willy@infradead.org>,
	"Alexander Viro" <viro@ftp.linux.org.uk>,
	Christoph Hellwig <hch@infradead.org>,
	"Pekka Enberg" <penberg@cs.helsinki.fi>,
	David Rientjes <rientjes@google.com>,
	Joonsoo Kim <iamjoonsoo.kim@lge.com>,
	Christopher Lameter <cl@linux.com>,
	Miklos Szeredi <mszeredi@redhat.com>,
	Andreas Dilger <adilger@dilger.ca>,
	Waiman Long <longman@redhat.com>, Tycho Andersen <tycho@tycho.ws>,
	"Theodore Ts'o" <tytso@mit.edu>, Andi Kleen <ak@linux.intel.com>,
	David Chinner <david@fromorbit.com>,
	Nick Piggin <npiggin@gmail.com>, Rik van Riel <riel@redhat.com>,
	Hugh Dickins <hughd@google.com>, Jonathan Corbet <corbet@lwn.net>,
	"linux-mm@kvack.org" <linux-mm@kvack.org>,
	"linux-fsdevel@vger.kernel.org" <linux-fsdevel@vger.kernel.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: [RFC PATCH v5 16/16] dcache: Add CONFIG_DCACHE_SMO
Date: Tue, 21 May 2019 02:05:38 +0000	[thread overview]
Message-ID: <20190521020530.GA18287@tower.DHCP.thefacebook.com> (raw)
In-Reply-To: <20190521013118.GB25898@eros.localdomain>

On Tue, May 21, 2019 at 11:31:18AM +1000, Tobin C. Harding wrote:
> On Tue, May 21, 2019 at 12:57:47AM +0000, Roman Gushchin wrote:
> > On Mon, May 20, 2019 at 03:40:17PM +1000, Tobin C. Harding wrote:
> > > In an attempt to make the SMO patchset as non-invasive as possible add a
> > > config option CONFIG_DCACHE_SMO (under "Memory Management options") for
> > > enabling SMO for the DCACHE.  Whithout this option dcache constructor is
> > > used but no other code is built in, with this option enabled slab
> > > mobility is enabled and the isolate/migrate functions are built in.
> > > 
> > > Add CONFIG_DCACHE_SMO to guard the partial shrinking of the dcache via
> > > Slab Movable Objects infrastructure.
> > 
> > Hm, isn't it better to make it a static branch? Or basically anything
> > that allows switching on the fly?
> 
> If that is wanted, turning SMO on and off per cache, we can probably do
> this in the SMO code in SLUB.

Not necessarily per cache, but without recompiling the kernel.
> 
> > It seems that the cost of just building it in shouldn't be that high.
> > And the question if the defragmentation worth the trouble is so much
> > easier to answer if it's possible to turn it on and off without rebooting.
> 
> If the question is 'is defragmentation worth the trouble for the
> dcache', I'm not sure having SMO turned off helps answer that question.
> If one doesn't shrink the dentry cache there should be very little
> overhead in having SMO enabled.  So if one wants to explore this
> question then they can turn on the config option.  Please correct me if
> I'm wrong.

The problem with a config option is that it's hard to switch over.

So just to test your changes in production a new kernel should be built,
tested and rolled out to a representative set of machines (which can be
measured in thousands of machines). Then if results are questionable,
it should be rolled back.

What you're actually guarding is the kmem_cache_setup_mobility() call,
which can be perfectly avoided using a boot option, for example. Turning
it on and off completely dynamic isn't that hard too.

Of course, it's up to you, it's just probably easier to find new users
of a new feature, when it's easy to test it.

Thanks!

  reply	other threads:[~2019-05-21  2:06 UTC|newest]

Thread overview: 32+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-05-20  5:40 [RFC PATCH v5 00/16] Slab Movable Objects (SMO) Tobin C. Harding
2019-05-20  5:40 ` [RFC PATCH v5 01/16] slub: Add isolate() and migrate() methods Tobin C. Harding
2019-05-21  0:37   ` Roman Gushchin
2019-05-20  5:40 ` [RFC PATCH v5 02/16] tools/vm/slabinfo: Add support for -C and -M options Tobin C. Harding
2019-05-20  5:40 ` [RFC PATCH v5 03/16] slub: Sort slab cache list Tobin C. Harding
2019-05-21  0:38   ` Roman Gushchin
2019-05-20  5:40 ` [RFC PATCH v5 04/16] slub: Slab defrag core Tobin C. Harding
2019-05-21  0:51   ` Roman Gushchin
2019-05-21  1:15     ` Tobin C. Harding
2019-05-21  1:25       ` Roman Gushchin
2019-05-20  5:40 ` [RFC PATCH v5 05/16] tools/vm/slabinfo: Add remote node defrag ratio output Tobin C. Harding
2019-05-20  5:40 ` [RFC PATCH v5 06/16] tools/vm/slabinfo: Add defrag_used_ratio output Tobin C. Harding
2019-05-20  5:40 ` [RFC PATCH v5 07/16] tools/testing/slab: Add object migration test module Tobin C. Harding
2019-05-20  5:40 ` [RFC PATCH v5 08/16] tools/testing/slab: Add object migration test suite Tobin C. Harding
2019-05-20  5:40 ` [RFC PATCH v5 09/16] lib: Separate radix_tree_node and xa_node slab cache Tobin C. Harding
2019-05-20  5:40 ` [RFC PATCH v5 10/16] xarray: Implement migration function for xa_node objects Tobin C. Harding
2019-05-20  5:40 ` [RFC PATCH v5 11/16] tools/testing/slab: Add XArray movable objects tests Tobin C. Harding
2019-05-20  5:40 ` [RFC PATCH v5 12/16] slub: Enable moving objects to/from specific nodes Tobin C. Harding
2019-05-20  5:40 ` [RFC PATCH v5 13/16] slub: Enable balancing slabs across nodes Tobin C. Harding
2019-05-21  1:04   ` Roman Gushchin
2019-05-21  1:44     ` Tobin C. Harding
2019-05-20  5:40 ` [RFC PATCH v5 14/16] dcache: Provide a dentry constructor Tobin C. Harding
2019-05-20  5:40 ` [RFC PATCH v5 15/16] dcache: Implement partial shrink via Slab Movable Objects Tobin C. Harding
2019-05-20  5:40 ` [RFC PATCH v5 16/16] dcache: Add CONFIG_DCACHE_SMO Tobin C. Harding
2019-05-21  0:57   ` Roman Gushchin
2019-05-21  1:31     ` Tobin C. Harding
2019-05-21  2:05       ` Roman Gushchin [this message]
2019-05-21  3:15         ` Tobin C. Harding
2019-05-29  3:54         ` Tobin C. Harding
2019-05-29 16:16           ` Roman Gushchin
2019-06-03  4:26             ` Tobin C. Harding
2019-06-03 20:34               ` Roman Gushchin

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