Re: RFC: Memory Tiering Kernel Interfaces (v3)

[Tim Chen]

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