linux-kernel - Re: [PATCH 2/2 v6] mm/mempolicy: Don't create weight sysfs for memoryless nodes

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Message-ID: <20250319095630.1075-1-yunjeong.mun@sk.com>
Date: Wed, 19 Mar 2025 18:56:21 +0900
From: Yunjeong Mun <yunjeong.mun@...com>
To: Gregory Price <gourry@...rry.net>
Cc: kernel_team@...ynix.com,
	Joshua Hahn <joshua.hahnjy@...il.com>,
	harry.yoo@...cle.com,
	ying.huang@...ux.alibaba.com,
	gregkh@...uxfoundation.org,
	rakie.kim@...com,
	akpm@...ux-foundation.org,
	rafael@...nel.org,
	lenb@...nel.org,
	dan.j.williams@...el.com,
	Jonathan.Cameron@...wei.com,
	dave.jiang@...el.com,
	horen.chuang@...ux.dev,
	hannes@...xchg.org,
	linux-kernel@...r.kernel.org,
	linux-acpi@...r.kernel.org,
	linux-mm@...ck.org,
	kernel-team@...a.com,
	Honggyu Kim <honggyu.kim@...com>
Subject: Re: [PATCH 2/2 v6] mm/mempolicy: Don't create weight sysfs for memoryless nodes

Hi Gregory, thanks for your kind explanation.

On Tue, 18 Mar 2025 11:13:13 -0400 Gregory Price <gourry@...rry.net> wrote:
> On Tue, Mar 18, 2025 at 08:02:46PM +0900, Honggyu Kim wrote:
> > 
> > 
> > On 3/18/2025 5:02 PM, Yunjeong Mun wrote:
> > 
> > Some simple corrections here.  host-bridge{0-3} above aren't detected from CEDT.
> > The corrected structure is as follows.
> > 
> > rootport/
> > ├── socket0
> > │   ├── cross-host-bridge0 -> SRAT && CEDT (interleave on) --> NODE 2
> > │   │   ├── host-bridge0
> > │   │   │   ├── cxl0 -> CEDT
>                           node 4
> > │   │   │   └── cxl1-> CEDT
>                           node 5
> > │   │   └── host-bridge1
> > │   │       ├── cxl2 -> CEDT
>                           node 6
> > │   │       └── cxl3 -> CEDT
>                           node 7
> > │   └── dram0 -> SRAT ---------------------------------------> NODE 0
> > └── socket1
> >      ├── cross-host-bridge1 -> SRAT && CEDT (interleave on)---> NODE 3
> >      │   ├── host-bridge2
> >      │   │   ├── cxl4 -> CEDT
>                            node 8
> >      │   │   └── cxl5 -> CEDT
>                            node 9
> >      │   └── host-bridge3
> >      │       ├── cxl6 -> CEDT
>                            node 10
> >      │       └── cxl7 -> CEDT
>                            node 11
> >      └── dram1 -> SRAT ---------------------------------------> NODE 1
> > 
> 
> This is correct and expected.
> 
> All of these nodes are "possible" depending on how the user decides to
> program the CXL decoders and expose memory to the page allocator.
> 
> In your /sys/bus/cxl/devices/ you should have something like
> 
>   decoder0.0  decoder0.1  decoder0.2   decoder0.3
>   decoder0.4  decoder0.5  decoder0.6   decoder0.7
>   decoder0.8  decoder0.9
> 

Yes, I can see many decoder#.# files in there, and their devtype values are
shown below:
    $ cat /sys/bus/cxl/devices/decoder*/devtype
    cxl_decoder_root
    ...
    cxl_decoder_switch
    ...
    cxl_decoder_endpoint

> These are the root decoders that should map up directly with each CEDT
> CFMWS entry.
> 
> 2 of them should have interleave settings.
> 
> If you were to then program the endpoint and hostbridge decoders with
> the matching non-interleave address values from the other CEDT entries,
> you could bring each individual device online in its own NUMA node.
> 

I think this means that I can program the endpoint(=cxl_decoder_endpoint)
to map to the 8 CFMWS, and the hostbridge decoder (=cxl_decoder switch) to map
to another 2 CFMWS(cross-host bridge).

> Or, you can do what you're doing now, and program the endpoints to map
> to the 2 cross-host bridge interleave root decoders.

In my understanding, that kind of programming is done at the firmware or BIOS 
layer, right?

> 
> So your platform is giving you the option of how to online your devices,
> and as such it needs to mark nodes as "possible" even if they're unused.
> 

Thank you for the clear explanation. I now understand why 'possible' has such
value.

> ~Gregory
> 

Best regards,
Yunjeong