linux-kernel - Re: [PATCH rfc 0/5] mm: introduce shrinker sysfs interface

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Date:   Tue, 19 Apr 2022 11:50:45 -0700
From:   Roman Gushchin <roman.gushchin@...ux.dev>
To:     Kent Overstreet <kent.overstreet@...il.com>
Cc:     linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
        Dave Chinner <dchinner@...hat.com>,
        linux-kernel@...r.kernel.org, Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>,
        Shakeel Butt <shakeelb@...gle.com>,
        Yang Shi <shy828301@...il.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH rfc 0/5] mm: introduce shrinker sysfs interface

On Tue, Apr 19, 2022 at 02:36:54PM -0400, Kent Overstreet wrote:
> On Fri, Apr 15, 2022 at 05:27:51PM -0700, Roman Gushchin wrote:
> > 7) Don't display cgroups with less than 500 attached objects
> >   $ echo 500 > count_memcg
> >   $ cat count_memcg
> >     53 817
> >     1868 886
> >     2396 799
> >     2462 861
> > 
> > 8) Don't display cgroups with less than 500 attached objects (sum over all nodes)
> >   $ echo "500" > count_memcg_node
> >   $ cat count_memcg_node
> >     53 810 7
> >     1868 886 0
> >     2396 799 0
> >     2462 861 0
> > 
> > 9) Scan system/root shrinker
> >   $ cat count
> >     212
> >   $ echo 100 > scan
> >   $ cat scan
> >     97
> >   $ cat count
> >     115
> 
> This part seems entirely overengineered though and a really bad idea - can we
> please _not_ store query state in the kernel? It's not thread safe, and it seems
> like overengineering before we've done the basics (just getting this stuff in
> sysfs is a major improvement!).

Yes, it's not great, but I don't have a better idea yet. How to return the number
of freed objects? Do you suggest to drop this functionality at all or there are
other options I'm not seeing?

Counting again isn't a good option either: new object could have been added to
the list during the scan.

Thanks!