Re: [RFC PATCH v5 16/16] dcache: Add CONFIG_DCACHE_SMO

From: "Tobin C. Harding" <me@tobin.cc>
To: Roman Gushchin <guro@fb.com>
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 13:15:08 +1000	[thread overview]
Message-ID: <20190521031508.GA31794@eros.localdomain> (raw)
In-Reply-To: <20190521020530.GA18287@tower.DHCP.thefacebook.com>

On Tue, May 21, 2019 at 02:05:38AM +0000, Roman Gushchin wrote:
> 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.

Ok, cool - I like it.  Will add for next version.

thanks,
Tobin.