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

From: Tobin C. Harding
Date: Tue May 28 2019 - 23:58:36 EST

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.

Hi Roman,

I've added a boot parameter to SLUB so that admins can enable/disable
SMO at boot time system wide. Then for each object that implements SMO
(currently XArray and dcache) I've also added a boot parameter to
enable/disable SMO for that cache specifically (these depend on SMO
being enabled system wide).

All three boot parameters default to 'off', I've added a config option
to default each to 'on'.

I've got a little more testing to do on another part of the set then the
PATCH version is coming at you :)

This is more a courtesy email than a request for comment, but please
feel free to shout if you don't like the method outlined above.

Fully dynamic config is not currently possible because currently the SMO
implementation does not support disabling mobility for a cache once it
is turned on, a bit of extra logic would need to be added and some state
stored - I'm not sure it warrants it ATM but that can be easily added
later if wanted. Maybe Christoph will give his opinion on this.