Date:   Tue, 19 Apr 2022 10:52:44 -0700
From:   Roman Gushchin <roman.gushchin@...ux.dev>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     linux-mm@...ck.org, Dave Chinner <dchinner@...hat.com>,
        linux-kernel@...r.kernel.org, Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>,
        Shakeel Butt <shakeelb@...gle.com>,
        Yang Shi <shy828301@...il.com>
Subject: Re: [PATCH rfc 0/5] mm: introduce shrinker sysfs interface

On Mon, Apr 18, 2022 at 09:27:09PM -0700, Andrew Morton wrote:
> On Fri, 15 Apr 2022 17:27:51 -0700 Roman Gushchin <roman.gushchin@...ux.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.
> 
> Please, "directory".

Of course, sorry :)

> 
> > 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.
> 
> I also was wondering "why not debugfs".

Fair enough, moving to debugfs in v1.

> 
> > Usage examples:
> > 
> > ...
> >
> > If the output doesn't fit into a single page, "...\n" is printed at the end of
> > output.
> 
> Unclear.  At the end of what output?

This is how it looks like when the output is too long:

[root@...50-1 sb-btrfs-24]# cat count_memcg
1 226
20 96
53 811
2429 2
218 13
581 29
911 124
1010 3
1043 1
1076 1
1241 60
1274 7
1307 39
1340 3
1406 14
1439 63
1472 54
1505 8
1538 1
1571 6
1604 39
1637 9
1670 8
1703 4
1736 1094
1802 2
1868 2
1901 52
1934 592
1967 32
			< CUT >
18797 1
18830 1
18863 1
18896 1
18929 1
18962 1
18995 1
19028 1
19061 1
19094 1
19127 1
19160 1
19193 1
...

I'll try to make it more obvious from the description.

> 
> > 
> > Roman Gushchin (5):
> >   mm: introduce sysfs interface for debugging kernel shrinker
> >   mm: memcontrol: introduce mem_cgroup_ino() and
> >     mem_cgroup_get_from_ino()
> >   mm: introduce memcg interfaces for shrinker sysfs
> >   mm: introduce numa interfaces for shrinker sysfs
> >   mm: provide shrinkers with names
> > 
> >  arch/x86/kvm/mmu/mmu.c                        |   2 +-
> >  ...
> >
> 
> Nothing under Documentation/!

I planned to add it after the rfc version. Will do.

Thank you for taking a look!

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