linux-kernel - Re: [PATCH v2 0/7] mm: introduce shrinker debugfs interface

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Message-ID: <Ymia75Bh/sn/FQdV@rh>
Date:   Wed, 27 Apr 2022 11:22:55 +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, Yang Shi <shy828301@...il.com>,
        Kent Overstreet <kent.overstreet@...il.com>,
        Hillf Danton <hdanton@...a.com>
Subject: Re: [PATCH v2 0/7] mm: introduce shrinker debugfs interface

On Tue, Apr 26, 2022 at 09:41:34AM -0700, Roman Gushchin wrote:
> Can you, please, summarize your position, because it's a bit unclear.
> You made a lot of good points about some details (e.g. shrinkers naming,
> and I totally agree there; machines with hundreds of nodes etc), then
> you said the active scanning is useless and then said the whole thing
> is useless and we're fine with what we have regarding shrinkers debugging.

Better introspection the first thing we need. Work on improving
that. I've been making suggestions to help improve introspection
infrastructure.

Before anything else, we need to improve introspection so we can
gain better insight into the problems we have. Once we understand
the problems better and have evidence to back up where the problems
lie and we have a plan to solve them, then we can talk about whether
we need other user accessible shrinker APIs.

For the moment, exposing shrinker control interfaces to userspace
could potentially be very bad because it exposes internal
architectural and implementation details to a user API.  Just
because it is in /sys/kernel/debug it doesn't mean applications
won't start to use it and build dependencies on it.

That doesn't mean I'm opposed to exposing a shrinker control
mechanism to debugfs - I'm still on the fence on that one. However,
I definitely think that an API that directly exposes the internal
implementation to userspace is the wrong way to go about this.

Fine grained shrinker control is not necessary to improve shrinker
introspection and OOM debugging capability, so if you want/need
control interfaces then I think you should separate those out into a
separate line of development where it doesn't derail the discussion
on how to improve shrinker/OOM introspection.

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