linux-kernel - Re: [RFC PATCH 0/3] Cgroup accounting of memory tier usage

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Date:   Wed, 15 Jun 2022 17:59:59 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     Tim Chen <tim.c.chen@...ux.intel.com>
Cc:     linux-mm@...ck.org, akpm@...ux-foundation.org,
        Wei Xu <weixugc@...gle.com>, Huang Ying <ying.huang@...el.com>,
        Greg Thelen <gthelen@...gle.com>,
        Yang Shi <shy828301@...il.com>,
        Davidlohr Bueso <dave@...olabs.net>,
        Brice Goglin <brice.goglin@...il.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Hesham Almatary <hesham.almatary@...wei.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Jonathan Cameron <Jonathan.Cameron@...wei.com>,
        Alistair Popple <apopple@...dia.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Feng Tang <feng.tang@...el.com>,
        Jagdish Gediya <jvgediya@...ux.ibm.com>,
        Baolin Wang <baolin.wang@...ux.alibaba.com>,
        David Rientjes <rientjes@...gle.com>,
        "Aneesh Kumar K . V" <aneesh.kumar@...ux.ibm.com>,
        Shakeel Butt <shakeelb@...gle.com>
Subject: Re: [RFC PATCH 0/3] Cgroup accounting of memory tier usage

On Wed 15-06-22 08:23:56, Tim Chen wrote:
> On Wed, 2022-06-15 at 13:11 +0200, Michal Hocko wrote:
[...]
> > > The patchset works with Aneesh's v6 memory-tiering implementation [1].
> > > It is a preparatory patch set before introducing features to
> > > control top tiered memory in cgroups.
> > > 
> > > I'll like to first get feedback to see if 
> > > (1) Controllng the topmost tiered memory is enough 
> > > or
> > > (2) Multiple tiers at the top levels need to be grouped into "toptier"
> > > or
> > > (3) There are use cases not covered by (1) and (2). 
> > 
> > I would start by asking why do we need a dedicated interface in the
> > first place. Why the existing numa_stat is not a proper interface. Right
> > now we only report LRU per node stats. Is this insufficient?
> > What is userspace expect to do based on the reported data?
> 
> Exporting the toptier information here is convenient for me for debugging
> purpose of seeing whether a cgroup's toptier usage is under control.
> Otherwise writing a script to parse numastat and the memtier heirachy will
> work too. Exporting toptier usage directly is optional and we don't have to do it.

Please keep in mind this is an userspace API which has to be maintained
for ever. We do not add those just to make debugging more convenient.

-- 
Michal Hocko
SUSE Labs