linux-kernel - Re: [PATCH 5/7] x86/resctrl: Add a new "snc_ways" file to the monitoring info directory.

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Date:   Fri, 3 Mar 2023 18:32:59 +0000
From:   James Morse <james.morse@....com>
To:     "Luck, Tony" <tony.luck@...el.com>,
        "Yu, Fenghua" <fenghua.yu@...el.com>,
        "Chatre, Reinette" <reinette.chatre@...el.com>,
        Peter Newman <peternewman@...gle.com>,
        Jonathan Corbet <corbet@....net>,
        "x86@...nel.org" <x86@...nel.org>
Cc:     Shaopeng Tan <tan.shaopeng@...itsu.com>,
        Jamie Iles <quic_jiles@...cinc.com>,
        Babu Moger <babu.moger@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
        "patches@...ts.linux.dev" <patches@...ts.linux.dev>
Subject: Re: [PATCH 5/7] x86/resctrl: Add a new "snc_ways" file to the
 monitoring info directory.

Hi Tony,

On 28/02/2023 17:44, Luck, Tony wrote:
>>> Make it easy for the user to tell if Sub-NUMA Cluster is enabled by
>>> providing an info/ file.
>>
>> I think what this is conveying to user-space is 'domain_id_is_numa_node'.
> 
> That seems more architecturally neutral. I like it.
> 
>> Does user-space need to know the number of ways?
> 
> I don't know. Maybe some might. Perhaps there is some better name that
> is architecturally neutral, but still has a numerical rather than boolean value?

If we don't know what user-space needs this for, I'd prefer we don't expose it. It'll be a
problem in the future if there is no single number we can put there.

I don't see what the difference between 2 and 4 would be used for when setting up resctrl,
unless you are choosing which numa-nodes to spread tasks over ... but the numa distance
would be a much better number to base that decision on.

User-space is able to perform the same calculation to find the snc_ways using the
cache/index stuff and node entries in sysfs.

>> Will this always be a single number, or will it ever be possible to have an SNC=2 and
>> SNC=1 package in the same system?
> 
> I sincerely hope that it is the same value across the system. Currently the
> BIOS setup option to enable SNC doesn't have per-socket choices, it is
> just an all-or-nothing choice. "2" isn't the only choice for number of SNC
> nodes on a socket. "4" is (or will be) a choice.

Yeah, in the arm world, partners get to make the decision on what is sane. Big-little
means someone could do something that looks like SNC in on cluster, but not another.

If we don't know what user-space needs it for, I'd prefer we don't expose it, just to
avoid giving out rope to shoot ourselves in the foot with.

Thanks,

James