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Date:   Mon, 18 Apr 2022 21:27:09 -0700
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Roman Gushchin <roman.gushchin@linux.dev>
Cc:     linux-mm@kvack.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
Message-Id: <20220418212709.42f2ba15e00999bb57086b27@linux-foundation.org>
In-Reply-To: <20220416002756.4087977-1-roman.gushchin@linux.dev>
References: <20220416002756.4087977-1-roman.gushchin@linux.dev>
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On Fri, 15 Apr 2022 17:27:51 -0700 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.

Please, "directory".

> 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".

> 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?

> 
> 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/!