Re: [PATCH resend] memcg: introduce per-memcg reclaim interface

From: Shakeel Butt <shakeelb@google.com>
To: Johannes Weiner <hannes@cmpxchg.org>
Cc: Roman Gushchin <roman.gushchin@linux.dev>,
	Yosry Ahmed <yosryahmed@google.com>,
	Michal Hocko <mhocko@kernel.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	David Rientjes <rientjes@google.com>, Tejun Heo <tj@kernel.org>,
	Zefan Li <lizefan.x@bytedance.com>,
	Cgroups <cgroups@vger.kernel.org>,
	"open list:DOCUMENTATION" <linux-doc@vger.kernel.org>,
	LKML <linux-kernel@vger.kernel.org>,
	Linux-MM <linux-mm@kvack.org>, Jonathan Corbet <corbet@lwn.net>,
	Yu Zhao <yuzhao@google.com>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	Wei Xu <weixugc@google.com>, Greg Thelen <gthelen@google.com>
Subject: Re: [PATCH resend] memcg: introduce per-memcg reclaim interface
Date: Mon, 4 Apr 2022 10:14:36 -0700	[thread overview]
Message-ID: <CALvZod466jLN8Bb67X9=iF5TvjOo-oEsHA-mGhpHnywGCP+WvA@mail.gmail.com> (raw)
In-Reply-To: <Ykdz7fiMFZeFltYy@cmpxchg.org>

On Fri, Apr 1, 2022 at 2:51 PM Johannes Weiner <hannes@cmpxchg.org> wrote:
>
> On Fri, Apr 01, 2022 at 02:21:52PM -0700, Roman Gushchin wrote:
> > > On Apr 1, 2022, at 2:13 PM, Johannes Weiner <hannes@cmpxchg.org> wrote:
> > >
> > > On Fri, Apr 01, 2022 at 11:39:30AM -0700, Roman Gushchin wrote:
> > >> The interface you're proposing is not really extensible, so we'll likely need to
> > >> introduce a new interface like memory.reclaim_ext very soon. Why not create
> > >> an extensible API from scratch?
> > >>
> > >> I'm looking at cgroup v2 documentation which describes various interface files
> > >> formats and it seems like given the number of potential optional arguments
> > >> the best option is nested keyed (please, refer to the Interface Files section).
> > >>
> > >> E.g. the format can be:
> > >> echo "1G type=file nodemask=1-2 timeout=30s" > memory.reclaim
> > >
> > > Yeah, that syntax looks perfect.
> > >
> > > But why do you think it's not extensible from the current patch? We
> > > can add those arguments one by one as we agree on them, and return
> > > -EINVAL if somebody passes an unknown parameter.
> > >
> > > It seems to me the current proposal is forward-compatible that way
> > > (with the current set of keyword pararms being the empty set :-))
> >
> > It wasn’t obvious to me. We spoke about positional arguments and then it wasn’t clear how to add them in a backward-compatible way. The last thing we want is a bunch of memory.reclaim* interfaces :)
> >
> > So yeah, let’s just describe it properly in the documentation, no code changes are needed.
>
> Sounds good to me!

To summarize for next version:

1) Add selftests.
2) Add documentation for potential future extension, so whoever adds
those features in future should follow the key-value format Roman is
suggesting.

Yosry, once we have agreement on the return value, please send the
next version resolving these three points.