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Date:   Tue, 19 Apr 2022 14:20:30 -0400
From:   Kent Overstreet <kent.overstreet@gmail.com>
To:     Roman Gushchin <roman.gushchin@linux.dev>
Cc:     linux-mm@kvack.org, Andrew Morton <akpm@linux-foundation.org>,
        Dave Chinner <dchinner@redhat.com>,
        linux-kernel@vger.kernel.org, Johannes Weiner <hannes@cmpxchg.org>,
        Michal Hocko <mhocko@kernel.org>,
        Shakeel Butt <shakeelb@google.com>,
        Yang Shi <shy828301@gmail.com>
Subject: Re: [PATCH rfc 0/5] mm: introduce shrinker sysfs interface
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References: <20220416002756.4087977-1-roman.gushchin@linux.dev>
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On Fri, Apr 15, 2022 at 05:27:51PM -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/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.

Cool!

I've been starting to sketch out some shrinker improvements of my own, perhaps
we could combine efforts. The issue I've been targeting is that when we hit an
OOM, we currently don't get a lot of useful information - shrinkers ought to be
included, and we really want information on shrinker's internal state (e.g.
object dirtyness) if we're to have a chance at understanding why memory isn't
getting reclaimed.

https://evilpiepirate.org/git/bcachefs.git/log/?h=shrinker_to_text

This adds a .to_text() method - a pretty-printer - that shrinkers can
implement, and then on OOM we report on the top 10 shrinkers by memory usage, in
sorted order.

Another thing I'd like to do is have shrinkers report usage not just in object
counts but in bytes; I think it should be obvious why that's desirable.

Maybe we could have a memory-reporting-and-shrinker-improvements session at LSF?
I'd love to do some collective brainstorming and get some real momementum going
in this area.