linux-kernel - Re: RFC: Memory Tiering Kernel Interfaces (v2)

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Date:   Thu, 12 May 2022 15:18:27 +0800
From:   "ying.huang@...el.com" <ying.huang@...el.com>
To:     Aneesh Kumar K V <aneesh.kumar@...ux.ibm.com>,
        Wei Xu <weixugc@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Greg Thelen <gthelen@...gle.com>,
        Yang Shi <shy828301@...il.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Jagdish Gediya <jvgediya@...ux.ibm.com>,
        Michal Hocko <mhocko@...nel.org>,
        Tim C Chen <tim.c.chen@...el.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Alistair Popple <apopple@...dia.com>,
        Baolin Wang <baolin.wang@...ux.alibaba.com>,
        Feng Tang <feng.tang@...el.com>,
        Jonathan Cameron <Jonathan.Cameron@...wei.com>,
        Davidlohr Bueso <dave@...olabs.net>,
        Dan Williams <dan.j.williams@...el.com>,
        David Rientjes <rientjes@...gle.com>,
        Linux MM <linux-mm@...ck.org>,
        Brice Goglin <brice.goglin@...il.com>,
        Hesham Almatary <hesham.almatary@...wei.com>
Subject: Re: RFC: Memory Tiering Kernel Interfaces (v2)

On Thu, 2022-05-12 at 12:42 +0530, Aneesh Kumar K V wrote:
> On 5/12/22 12:33 PM, ying.huang@...el.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

This will make the interface not extensible.  Even a single file
"/sys/devices/system/node/memtiers" is better.  As an readonly file, it
should be OK to put multiple values in it.

I remember that one rule for sysfs is that it is accessed more via
libsysfs.  Does that make life easier?

Best Regards,
Huang, Ying