Re: [PATCH rfc 0/5] mm: introduce shrinker sysfs interface

[Roman Gushchin <roman.gushchin@linux.dev>]

On Wed, Apr 20, 2022 at 03:24:49PM -0700, Yang Shi wrote:
> On Fri, Apr 15, 2022 at 5:28 PM Roman Gushchin <roman.gushchin@linux.dev> wrote:
> >
> > There are 50+ different shrinkers in the kernel, many with their own bells and
> > whistles. Under the memory pressure the kernel applies some pressure on each of
> > them in the order of which they were created/registered in the system. Some
> > of them can contain only few objects, some can be quite large. Some can be
> > effective at reclaiming memory, some not.
> >
> > The only existing debugging mechanism is a couple of tracepoints in
> > do_shrink_slab(): mm_shrink_slab_start and mm_shrink_slab_end. They aren't
> > covering everything though: shrinkers which report 0 objects will never show up,
> > there is no support for memcg-aware shrinkers. Shrinkers are identified by their
> > scan function, which is not always enough (e.g. hard to guess which super
> > block's shrinker it is having only "super_cache_scan"). They are a passive
> > mechanism: there is no way to call into counting and scanning of an individual
> > shrinker and profile it.
> >
> > To provide a better visibility and debug options for memory shrinkers
> > this patchset introduces a /sys/kernel/shrinker interface, to some extent
> > similar to /sys/kernel/slab.
> >
> > For each shrinker registered in the system a folder is created. The folder
> > contains "count" and "scan" files, which allow to trigger count_objects()
> > and scan_objects() callbacks. For memcg-aware and numa-aware shrinkers
> > count_memcg, scan_memcg, count_node, scan_node, count_memcg_node
> > and scan_memcg_node are additionally provided. They allow to get per-memcg
> > and/or per-node object count and shrink only a specific memcg/node.
> >
> > To make debugging more pleasant, the patchset also names all shrinkers,
> > so that sysfs entries can have more meaningful names.
> >
> > Usage examples:
> 
> Thanks, Roman. A follow-up question, why do we have to implement this
> in kernel if we just count the objects? It seems userspace tools could
> achieve it too, for example, drgn :-). Actually I did write a drgn
> script for debugging a problem a few months ago, which iterates
> specific memcg's lru_list to count the objects by their state.

Good question! It's because not all shrinkers are lru_list-based
and even some lru_list-based are implementing a custom logic on top of it,
e.g. shadow nodes. So there is no simple way to get the count from
a generic shrinker.

Also I want to be able to reclaim individual shrinkers from userspace
(e.g. to profile how effective the shrinking is).

Thanks!