linux-kernel - Re: [PATCH 0/7] ACPI HMAT memory sysfs representation

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Message-ID: <aae34dde-fa70-870a-9b74-fff9e385bfc9@arm.com>
Date:   Thu, 22 Nov 2018 17:22:19 +0530
From:   Anshuman Khandual <anshuman.khandual@....com>
To:     Dave Hansen <dave.hansen@...el.com>,
        Keith Busch <keith.busch@...el.com>,
        linux-kernel@...r.kernel.org, linux-acpi@...r.kernel.org,
        linux-mm@...ck.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Rafael Wysocki <rafael@...nel.org>,
        Dan Williams <dan.j.williams@...el.com>
Subject: Re: [PATCH 0/7] ACPI HMAT memory sysfs representation



On 11/19/2018 11:07 PM, Dave Hansen wrote:
> On 11/18/18 9:44 PM, Anshuman Khandual wrote:
>> IIUC NUMA re-work in principle involves these functional changes
>>
>> 1. Enumerating compute and memory nodes in heterogeneous environment (short/medium term)
> 
> This patch set _does_ that, though.
> 
>> 2. Enumerating memory node attributes as seen from the compute nodes (short/medium term)
> 
> It does that as well (a subset at least).
> 
> It sounds like the subset that's being exposed is insufficient for yo
> We did that because we think doing anything but a subset in sysfs will
> just blow up sysfs:  MAX_NUMNODES is as high as 1024, so if we have 4
> attributes, that's at _least_ 1024*1024*4 files if we expose *all*
> combinations.
Each permutation need not be a separate file inside all possible NODE X
(/sys/devices/system/node/nodeX) directories. It can be a top level file
enumerating various attribute values for a given (X, Y) node pair based
on an offset something like /proc/pid/pagemap.

> 
> Do we agree that sysfs is unsuitable for exposing attributes in this manner?
> 

Yes, for individual files. But this can be worked around with an offset
based access from a top level global attributes file as mentioned above.
Is there any particular advantage of using individual files for each
given attribute ? I was wondering that a single unsigned long (u64) will
be able to pack 8 different attributes where each individual attribute
values can be abstracted out in 8 bits.