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

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Message-ID: <YocaGTFTB4qLuSw3@rh>
Date:   Fri, 20 May 2022 14:33:29 +1000
From:   Dave Chinner <dchinner@...hat.com>
To:     Roman Gushchin <roman.gushchin@...ux.dev>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        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 Thu, May 19, 2022 at 10:15:04AM -0700, Roman Gushchin wrote:
> 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?

I have no time available to look at this right now, and won't for a
while.

Cheers,

Dave.
-- 
Dave Chinner
dchinner@...hat.com