linux-kernel - Re: [PATCH v1 0/5] mm: introduce shrinker debugfs interface

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Date:   Fri, 22 Apr 2022 07:57:55 -0700
From:   Roman Gushchin <roman.gushchin@...ux.dev>
To:     Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org
Cc:     Dave Chinner <dchinner@...hat.com>, linux-kernel@...r.kernel.org,
        Yang Shi <shy828301@...il.com>,
        Kent Overstreet <kent.overstreet@...il.com>,
        Hillf Danton <hdanton@...a.com>
Subject: Re: [PATCH v1 0/5] mm: introduce shrinker debugfs interface

On Thu, Apr 21, 2022 at 06:58:48PM -0700, Roman Gushchin 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/debug/shrinker interface, to some extent
> similar to /sys/kernel/slab.
> 
> For each shrinker registered in the system a directory is created. The directory
> 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 debugfs entries can have more meaningful names.
> 
> v1:
>   1) switched to debugfs, suggested by Mike, Andrew, Greg and others
>   2) switched to seq_file API for output, no PAGE_SIZE limit anymore, by Andrew
>   3) switched to down_read_killable(), suggested by Hillf
>   4) dropped stateful filtering and "freed" returning, by Kent

>   5) added docs, by Andrew

Not quite true, gonna add it and also a cgroup tree inspection tool
and send out v2.

Thanks!