linux-kernel - Re: RFC: Memory Tiering Kernel Interfaces (v3)

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Date:   Tue, 07 Jun 2022 12:25:42 -0700
From:   Tim Chen <tim.c.chen@...ux.intel.com>
To:     Jonathan Cameron <Jonathan.Cameron@...wei.com>,
        Ying Huang <ying.huang@...el.com>
Cc:     Wei Xu <weixugc@...gle.com>,
        Aneesh Kumar K V <aneesh.kumar@...ux.ibm.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Greg Thelen <gthelen@...gle.com>,
        Yang Shi <shy828301@...il.com>,
        Davidlohr Bueso <dave@...olabs.net>,
        Tim C Chen <tim.c.chen@...el.com>,
        Brice Goglin <brice.goglin@...il.com>,
        Michal Hocko <mhocko@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Hesham Almatary <hesham.almatary@...wei.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Alistair Popple <apopple@...dia.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Feng Tang <feng.tang@...el.com>, Linux MM <linux-mm@...ck.org>,
        Jagdish Gediya <jvgediya@...ux.ibm.com>,
        Baolin Wang <baolin.wang@...ux.alibaba.com>,
        David Rientjes <rientjes@...gle.com>
Subject: Re: RFC: Memory Tiering Kernel Interfaces (v3)

On Mon, 2022-05-30 at 13:50 +0100, Jonathan Cameron wrote:
> 
> > When discussed offline, Tim Chen pointed out that with the proposed
> > interface, it's unconvenient to know the position of a given memory tier
> > in all memory tiers.  We must sort "rank" of all memory tiers to know
> > that.  "possible" file can be used for that.  Although "possible" file
> > can be generated with a shell script, it's more convenient to show it
> > directly.
> > 
> > Another way to address the issue is to add memtierN/pos for each memory
> > tier as suggested by Tim.  It's readonly and will show position of
> > "memtierN" in all memory tiers.  It's even better to show the relative
> > postion to the default memory tier (DRAM with CPU). That is, the
> > position of DRAM memory tier is 0.
> > 
> > Unlike memory tier device ID or rank, the position is relative and
> > dynamic.
> 
> Hi,
> 
> I'm unconvinced.  This is better done with a shell script than
> by adding ABI we'll have to live with for ever..
> 
> I'm no good at shell scripting but this does the job 
> grep "" tier*/rank | sort -n -k 2 -t : 
> 
> tier2/rank:50
> tier0/rank:100
> tier1/rank:200
> tier3/rank:240
> 
> I'm sure someone more knowledgeable will do it in a simpler fashion still.
> 
> 

You can argue that 

$ cat /sys/devices/system/cpu/cpu1/topology/core_siblings
f
$ cat /sys/devices/system/cpu/cpu1/topology/core_siblings_list
0-3

provide exactly the same information and we should get rid of
core_siblings_list.  I think core_siblings_list exists to make
it easier for a human, so he/she doesn't have to parse the mask,
or write a script to find out the ids of CPUs who are siblings.

I think in the same spirit, having an interface to allow a
human to quickly see the hierachical relationship of tiers 
relative to each other is helpful. 

Tim