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From:   Wei Xu <weixugc@google.com>
Date:   Thu, 12 May 2022 00:22:33 -0700
Message-ID: <CAAPL-u-g86QqHaHGGtVJMER8ENC2dpekK+2qOkxoRFmC0F_80g@mail.gmail.com>
Subject: Re: RFC: Memory Tiering Kernel Interfaces (v2)
To:     Aneesh Kumar K V <aneesh.kumar@linux.ibm.com>
Cc:     "ying.huang@intel.com" <ying.huang@intel.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Greg Thelen <gthelen@google.com>,
        Yang Shi <shy828301@gmail.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Jagdish Gediya <jvgediya@linux.ibm.com>,
        Michal Hocko <mhocko@kernel.org>,
        Tim C Chen <tim.c.chen@intel.com>,
        Dave Hansen <dave.hansen@intel.com>,
        Alistair Popple <apopple@nvidia.com>,
        Baolin Wang <baolin.wang@linux.alibaba.com>,
        Feng Tang <feng.tang@intel.com>,
        Jonathan Cameron <Jonathan.Cameron@huawei.com>,
        Davidlohr Bueso <dave@stgolabs.net>,
        Dan Williams <dan.j.williams@intel.com>,
        David Rientjes <rientjes@google.com>,
        Linux MM <linux-mm@kvack.org>,
        Brice Goglin <brice.goglin@gmail.com>,
        Hesham Almatary <hesham.almatary@huawei.com>
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On Thu, May 12, 2022 at 12:12 AM Aneesh Kumar K V
<aneesh.kumar@linux.ibm.com> wrote:
>
> On 5/12/22 12:33 PM, ying.huang@intel.com wrote:
> > On Wed, 2022-05-11 at 23:22 -0700, Wei Xu wrote:
> >> Sysfs Interfaces
> >> ================
> >>
> >> * /sys/devices/system/memtier/memtierN/nodelist
> >>
> >>    where N = 0, 1, 2 (the kernel supports only 3 tiers for now).
> >>
> >>    Format: node_list
> >>
> >>    Read-only.  When read, list the memory nodes in the specified tier.
> >>
> >>    Tier 0 is the highest tier, while tier 2 is the lowest tier.
> >>
> >>    The absolute value of a tier id number has no specific meaning.
> >>    What matters is the relative order of the tier id numbers.
> >>
> >>    When a memory tier has no nodes, the kernel can hide its memtier
> >>    sysfs files.
> >>
> >> * /sys/devices/system/node/nodeN/memtier
> >>
> >>    where N = 0, 1, ...
> >>
> >>    Format: int or empty
> >>
> >>    When read, list the memory tier that the node belongs to.  Its value
> >>    is empty for a CPU-only NUMA node.
> >>
> >>    When written, the kernel moves the node into the specified memory
> >>    tier if the move is allowed.  The tier assignment of all other nodes
> >>    are not affected.
> >>
> >>    Initially, we can make this interface read-only.
> >
> > It seems that "/sys/devices/system/node/nodeN/memtier" has all
> > information we needed.  Do we really need
> > "/sys/devices/system/memtier/memtierN/nodelist"?
> >
> > That can be gotten via a simple shell command line,
> >
> > $ grep . /sys/devices/system/node/nodeN/memtier | sort -n -k 2 -t ':'
> >
>
> It will be really useful to fetch the memory tier node list in an easy
> fashion rather than reading multiple sysfs directories. If we don't have
> other attributes for memorytier, we could keep
> "/sys/devices/system/memtier/memtierN" a NUMA node list there by
> avoiding /sys/devices/system/memtier/memtierN/nodelist
>
> -aneesh

It is harder to implement memtierN as just a file and doesn't follow
the existing sysfs pattern, either.  Besides, it is extensible to have
memtierN as a directory.