Message-ID: <20190312175958.GA31407@tower.DHCP.thefacebook.com>
Date:   Tue, 12 Mar 2019 18:00:09 +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>,
        Christopher Lameter <cl@...ux.com>,
        Pekka Enberg <penberg@...helsinki.fi>,
        Matthew Wilcox <willy@...radead.org>,
        "Tycho Andersen" <tycho@...ho.ws>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC 04/15] slub: Enable Slab Movable Objects (SMO)

On Tue, Mar 12, 2019 at 12:47:12PM +1100, Tobin C. Harding wrote:
> On Mon, Mar 11, 2019 at 10:48:45PM +0000, Roman Gushchin wrote:
> > On Fri, Mar 08, 2019 at 03:14:15PM +1100, Tobin C. Harding wrote:
> > > We have now in place a mechanism for adding callbacks to a cache in
> > > order to be able to implement object migration.
> > > 
> > > Add a function __move() that implements SMO by moving all objects in a
> > > slab page using the isolate/migrate callback methods.
> > > 
> > > Co-developed-by: Christoph Lameter <cl@...ux.com>
> > > Signed-off-by: Tobin C. Harding <tobin@...nel.org>
> > > ---
> > >  mm/slub.c | 85 +++++++++++++++++++++++++++++++++++++++++++++++++++++++
> > >  1 file changed, 85 insertions(+)
> > > 
> > > diff --git a/mm/slub.c b/mm/slub.c
> > > index 0133168d1089..6ce866b420f1 100644
> > > --- a/mm/slub.c
> > > +++ b/mm/slub.c
> > > @@ -4325,6 +4325,91 @@ int __kmem_cache_create(struct kmem_cache *s, slab_flags_t flags)
> > >  	return err;
> > >  }
> > >  
> > > +/*
> > > + * Allocate a slab scratch space that is sufficient to keep pointers to
> > > + * individual objects for all objects in cache and also a bitmap for the
> > > + * objects (used to mark which objects are active).
> > > + */
> > > +static inline void *alloc_scratch(struct kmem_cache *s)
> > > +{
> > > +	unsigned int size = oo_objects(s->max);
> > > +
> > > +	return kmalloc(size * sizeof(void *) +
> > > +		       BITS_TO_LONGS(size) * sizeof(unsigned long),
> > > +		       GFP_KERNEL);
> > 
> > I wonder how big this allocation can be?
> > Given that the reason for migration is probably highly fragmented memory,
> > we probably don't want to have a high-order allocation here. So maybe
> > kvmalloc()?
> > 
> > > +}
> > > +
> > > +/*
> > > + * __move() - Move all objects in the given slab.
> > > + * @page: The slab we are working on.
> > > + * @scratch: Pointer to scratch space.
> > > + * @node: The target node to move objects to.
> > > + *
> > > + * If the target node is not the current node then the object is moved
> > > + * to the target node.  If the target node is the current node then this
> > > + * is an effective way of defragmentation since the current slab page
> > > + * with its object is exempt from allocation.
> > > + */
> > > +static void __move(struct page *page, void *scratch, int node)
> > > +{
> > 
> > __move() isn't a very explanatory name. kmem_cache_move() (as in Christopher's
> > version) is much better, IMO. Or maybe move_slab_objects()?
> 
> How about move_slab_page()?  We use kmem_cache_move() later in the
> series.  __move() moves all objects in the given page but not all
> objects in this cache (which kmem_cache_move() later does).  Open to
> further suggestions though, naming things is hard :)
> 
> Christopher's original patch uses kmem_cache_move() for a function that
> only moves objects from within partial slabs, I changed it because I did
> not think this name suitably describes the behaviour.  So from the
> original I rename:
> 
> 	__move() -> __defrag()
> 	kmem_cache_move() -> __move()
> 	
> And reuse kmem_cache_move() for move _all_ objects (includes full list).
> 
> With this set applied we have the call chains
> 
> kmem_cache_shrink()		# Defined in slab_common.c, exported to kernel.
>  -> __kmem_cache_shrink()	# Defined in slub.c
>    -> __defrag()		# Unconditionally (i.e 100%)
>      -> __move()
> 
> kmem_cache_defrag()		# Exported to kernel
>  -> __defrag()
>    -> __move()
> 
> move_store()			# sysfs
>  -> kmem_cache_move()
>    -> __move()
>  or
>  -> __move_all_objects_to()
>    -> kmem_cache_move()
>      -> __move()
> 
> 
> Suggested improvements?

move_slab_page() looks good to me. I don't have a strong opinion on naming here,
I just think that unique names are always preferred even for static functions
(that's why I don't like __move()). Also, it's not clear what '__' means here.
So maybe move_slab_page(), defrag_kmem_node(), etc? Or something like this.

> 
> > Also, it's usually better to avoid adding new functions without calling them.
> > Maybe it's possible to merge this patch with (9)?
> 
> Understood.  The reason behind this is that I attempted to break this
> set up separating the implementation of SMO with the addition of each
> feature.  This function is only called when features are implemented to
> use SMO, I did not want to elevate any feature above any other by
> including it in this patch.  I'm open to suggestions on how to order
> though, also I'm happy to order it differently if/when we do PATCH
> version.  Is that acceptable for the RFC versions?

The general rule is that any patch should be self-sufficient.
If it's required to look into further patches to understand why something
has been done, it makes the review process harder.

I think it's possible to make the patchset to conform this rule with
some almost cosmetic changes, e.g. move some changes from one patch
to another, add some comments, etc.

Anyway, it's definitely fine for an RFC, just something to think of
in the next version.

Thanks!

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