linux-kernel - Re: [PATCHv4 10/13] node: Add memory caching attributes

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Date:   Tue, 12 Feb 2019 09:11:03 +0100
From:   Brice Goglin <Brice.Goglin@...ia.fr>
To:     Keith Busch <keith.busch@...el.com>,
        Jonathan Cameron <jonathan.cameron@...wei.com>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-acpi@...r.kernel.org" <linux-acpi@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Rafael Wysocki <rafael@...nel.org>,
        "Hansen, Dave" <dave.hansen@...el.com>,
        "Williams, Dan J" <dan.j.williams@...el.com>
Subject: Re: [PATCHv4 10/13] node: Add memory caching attributes


Le 11/02/2019 à 16:23, Keith Busch a écrit :
> On Sun, Feb 10, 2019 at 09:19:58AM -0800, Jonathan Cameron wrote:
>> On Sat, 9 Feb 2019 09:20:53 +0100
>> Brice Goglin <Brice.Goglin@...ia.fr> wrote:
>>
>>> Hello Keith
>>>
>>> Could we ever have a single side cache in front of two NUMA nodes ? I
>>> don't see a way to find that out in the current implementation. Would we
>>> have an "id" and/or "nodemap" bitmask in the sidecache structure ?
>> This is certainly a possible thing for hardware to do.
>>
>> ACPI IIRC doesn't provide any means of representing that - your best
>> option is to represent it as two different entries, one for each of the
>> memory nodes.  Interesting question of whether you would then claim
>> they were half as big each, or the full size.  Of course, there are
>> other possible ways to get this info beyond HMAT, so perhaps the interface
>> should allow it to be exposed if available?
> HMAT doesn't do this, but I want this interface abstracted enough from
> HMAT to express whatever is necessary.
>
> The CPU cache is the closest existing exported attributes to this,
> and they provide "shared_cpu_list". To that end, I can export a
> "shared_node_list", though previous reviews strongly disliked multi-value
> sysfs entries. :(
>
> Would shared-node symlinks capture the need, and more acceptable?


As a user-space guy reading these files/symlinks, I would prefer reading
a bitmask just like we do for CPU cache "cpumap" or CPU "siblings" files
(or sibling_list).

Reading a directory and looking for dentries matching "foo%d" is far
less convenient  in C. If all these files are inside a dedicated
subdirectory, it's better but still not as easy.

Brice