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Date:   Tue, 21 May 2019 02:05:38 +0000
From:   Roman Gushchin <guro@...com>
To:     "Tobin C. Harding" <me@...in.cc>
CC:     "Tobin C. Harding" <tobin@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Matthew Wilcox <willy@...radead.org>,
        "Alexander Viro" <viro@....linux.org.uk>,
        Christoph Hellwig <hch@...radead.org>,
        "Pekka Enberg" <penberg@...helsinki.fi>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Christopher Lameter <cl@...ux.com>,
        Miklos Szeredi <mszeredi@...hat.com>,
        Andreas Dilger <adilger@...ger.ca>,
        Waiman Long <longman@...hat.com>,
        Tycho Andersen <tycho@...ho.ws>,
        "Theodore Ts'o" <tytso@....edu>, Andi Kleen <ak@...ux.intel.com>,
        David Chinner <david@...morbit.com>,
        Nick Piggin <npiggin@...il.com>,
        Rik van Riel <riel@...hat.com>,
        Hugh Dickins <hughd@...gle.com>,
        Jonathan Corbet <corbet@....net>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH v5 16/16] dcache: Add CONFIG_DCACHE_SMO

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!

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