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

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Date:   Tue, 19 Apr 2022 11:43:34 -0700
From:   Roman Gushchin <roman.gushchin@...ux.dev>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     linux-mm@...ck.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>
Subject: Re: [PATCH rfc 0/5] mm: introduce shrinker sysfs interface

On Tue, Apr 19, 2022 at 11:25:49AM -0700, Andrew Morton wrote:
> On Tue, 19 Apr 2022 10:52:44 -0700 Roman Gushchin <roman.gushchin@...ux.dev> wrote:
> 
> > > Unclear.  At the end of what output?
> > 
> > This is how it looks like when the output is too long:
> > 
> > [root@...50-1 sb-btrfs-24]# cat count_memcg
> > 1 226
> > 20 96
> > 53 811
> > 2429 2
> > 218 13
> > 581 29
> > 911 124
> > 1010 3
> > 1043 1
> > 1076 1
> > 1241 60
> > 1274 7
> > 1307 39
> > 1340 3
> > 1406 14
> > 1439 63
> > 1472 54
> > 1505 8
> > 1538 1
> > 1571 6
> > 1604 39
> > 1637 9
> > 1670 8
> > 1703 4
> > 1736 1094
> > 1802 2
> > 1868 2
> > 1901 52
> > 1934 592
> > 1967 32
> > 			< CUT >
> > 18797 1
> > 18830 1
> 
> We do that in-kernel?  Why?  That just makes parsers harder to write?
> If someone has issues then direct them at /usr/bin/less?

It comes from the sysfs limitation: it expects that the output should fit
into the PAGE_SIZE. If the number of cgroups (and nodes) is large, it's not
always possible. In theory something like seq_file API should be used, but
Idk how hard it's to mix it with the sysfs/debugfs API. I'll try to figure
this out.