Re: [RFC PATCH v5 04/16] slub: Slab defrag core

[Roman Gushchin]

On Tue, May 21, 2019 at 11:15:25AM +1000, Tobin C. Harding wrote:
> On Tue, May 21, 2019 at 12:51:57AM +0000, Roman Gushchin wrote:
> > On Mon, May 20, 2019 at 03:40:05PM +1000, Tobin C. Harding wrote:
> > > Internal fragmentation can occur within pages used by the slub
> > > allocator.  Under some workloads large numbers of pages can be used by
> > > partial slab pages.  This under-utilisation is bad simply because it
> > > wastes memory but also because if the system is under memory pressure
> > > higher order allocations may become difficult to satisfy.  If we can
> > > defrag slab caches we can alleviate these problems.
> > > 
> > > Implement Slab Movable Objects in order to defragment slab caches.
> > > 
> > > Slab defragmentation may occur:
> > > 
> > > 1. Unconditionally when __kmem_cache_shrink() is called on a slab cache
> > >    by the kernel calling kmem_cache_shrink().
> > > 
> > > 2. Unconditionally through the use of the slabinfo command.
> > > 
> > > 	slabinfo <cache> -s
> > > 
> > > 3. Conditionally via the use of kmem_cache_defrag()
> > > 
> > > - Use Slab Movable Objects when shrinking cache.
> > > 
> > > Currently when the kernel calls kmem_cache_shrink() we curate the
> > > partial slabs list.  If object migration is not enabled for the cache we
> > > still do this, if however, SMO is enabled we attempt to move objects in
> > > partially full slabs in order to defragment the cache.  Shrink attempts
> > > to move all objects in order to reduce the cache to a single partial
> > > slab for each node.
> > > 
> > > - Add conditional per node defrag via new function:
> > > 
> > > 	kmem_defrag_slabs(int node).
> > > 
> > > kmem_defrag_slabs() attempts to defragment all slab caches for
> > > node. Defragmentation is done conditionally dependent on MAX_PARTIAL
> > > _and_ defrag_used_ratio.
> > > 
> > >    Caches are only considered for defragmentation if the number of
> > >    partial slabs exceeds MAX_PARTIAL (per node).
> > > 
> > >    Also, defragmentation only occurs if the usage ratio of the slab is
> > >    lower than the configured percentage (sysfs field added in this
> > >    patch).  Fragmentation ratios are measured by calculating the
> > >    percentage of objects in use compared to the total number of objects
> > >    that the slab page can accommodate.
> > > 
> > >    The scanning of slab caches is optimized because the defragmentable
> > >    slabs come first on the list. Thus we can terminate scans on the
> > >    first slab encountered that does not support defragmentation.
> > > 
> > >    kmem_defrag_slabs() takes a node parameter. This can either be -1 if
> > >    defragmentation should be performed on all nodes, or a node number.
> > > 
> > >    Defragmentation may be disabled by setting defrag ratio to 0
> > > 
> > > 	echo 0 > /sys/kernel/slab/<cache>/defrag_used_ratio
> > > 
> > > - Add a defrag ratio sysfs field and set it to 30% by default. A limit
> > > of 30% specifies that more than 3 out of 10 available slots for objects
> > > need to be in use otherwise slab defragmentation will be attempted on
> > > the remaining objects.
> > > 
> > > In order for a cache to be defragmentable the cache must support object
> > > migration (SMO).  Enabling SMO for a cache is done via a call to the
> > > recently added function:
> > > 
> > > 	void kmem_cache_setup_mobility(struct kmem_cache *,
> > > 				       kmem_cache_isolate_func,
> > > 			               kmem_cache_migrate_func);
> > > 
> > > Co-developed-by: Christoph Lameter <cl@xxxxxxxxx>
> > > Signed-off-by: Tobin C. Harding <tobin@xxxxxxxxxx>
> > > ---
> > >  Documentation/ABI/testing/sysfs-kernel-slab |  14 +
> > >  include/linux/slab.h                        |   1 +
> > >  include/linux/slub_def.h                    |   7 +
> > >  mm/slub.c                                   | 385 ++++++++++++++++----
> > >  4 files changed, 334 insertions(+), 73 deletions(-)
> > 
> > Hi Tobin!
> > 
> > Overall looks very good to me! I'll take another look when you'll post
> > a non-RFC version, but so far I can't find any issues.
> 
> Thanks for the reviews.
> 
> > A generic question: as I understand, you do support only root kmemcaches now.
> > Is kmemcg support in plans?
> 
> I know very little about cgroups, I have no plans for this work.
> However, I'm not the architect behind this - Christoph is guiding the
> direction on this one.  Perhaps he will comment.
> 
> > Without it the patchset isn't as attractive to anyone using cgroups,
> > as it could be. Also, I hope it can solve (or mitigate) the memcg-specific
> > problem of scattering vfs cache workingset over multiple generations of the
> > same cgroup (their kmem_caches).
> 
> I'm keen to work on anything that makes this more useful so I'll do some
> research.  Thanks for the idea.

You're welcome! I'm happy to help or even to do it by myself, once
your patches will be merged.

Thanks!