linux-kernel - Re: [PATCH v3 0/6] mm: introduce shrinker debugfs interface

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Message-ID: <YoZ7GA9cjQiylEP7@carbon>
Date:   Thu, 19 May 2022 10:15:04 -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,
        Kent Overstreet <kent.overstreet@...il.com>,
        Hillf Danton <hdanton@...a.com>,
        Christophe JAILLET <christophe.jaillet@...adoo.fr>
Subject: Re: [PATCH v3 0/6] mm: introduce shrinker debugfs interface

On Mon, May 09, 2022 at 11:38:14AM -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").
> 
> 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.
> As now, the directory will contain only a "scan" file, which allows to get
> the number of managed objects for each memory cgroup (for memcg-aware shrinkers)
> and each numa node (for numa-aware shrinkers on a numa machine). Other
> interfaces might be added in the future.
> 
> To make debugging more pleasant, the patchset also names all shrinkers,
> so that debugfs entries can have meaningful names.
> 
> 
> v3:
>   1) separated the "scan" part into a separate patch, by Dave
>   2) merged *_memcg, *_node and *_memcg_node interfaces, by Dave
>   3) shrinkers naming enhancements, by Christophe and Dave
>   4) added signal_pending() check, by Hillf
>   5) enabled by default, by Dave

Any comments? Thoughts? Objections?

Thanks!