linux-kernel - Re: [PATCH v5 8/8] selftests/resctrl: Adjust effective L3 cache size when SNC enabled

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Message-ID: <7gbxb6kxwa6anvcc4dquyuu7yronww6ztelazn2xllplorhbp5@i7o4fum5tjuf>
Date:   Tue, 19 Sep 2023 08:50:33 +0200
From:   Maciej Wieczór-Retman 
        <maciej.wieczor-retman@...el.com>
To:     "Luck, Tony" <tony.luck@...el.com>
CC:     "Shaopeng Tan (Fujitsu)" <tan.shaopeng@...itsu.com>,
        "Yu, Fenghua" <fenghua.yu@...el.com>,
        "Chatre, Reinette" <reinette.chatre@...el.com>,
        "Peter Newman" <peternewman@...gle.com>,
        Jonathan Corbet <corbet@....net>,
        Shuah Khan <skhan@...uxfoundation.org>,
        "x86@...nel.org" <x86@...nel.org>,
        James Morse <james.morse@....com>,
        Jamie Iles <quic_jiles@...cinc.com>,
        Babu Moger <babu.moger@....com>,
        Randy Dunlap <rdunlap@...radead.org>,
        "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 v5 8/8] selftests/resctrl: Adjust effective L3 cache size
 when SNC enabled

On 2023-09-07 at 16:19:37 +0000, Luck, Tony wrote:
>> > +   if (4 * node_cpus >= cache_cpus)
>> > +           return 4;
>> > +   else if (2 * node_cpus >= cache_cpus)
>> > +           return 2;
>>
>>
>> If "4 * node_cpus >= cache_cpus " is not true，
>> "2 * node_cpus >= cache_cpus" will never be true.
>> Is it the following code?
>>
>> +     if (2 * node_cpus >= cache_cpus)
>> +             return 2;
>> +     else if (4 * node_cpus >= cache_cpus)
>> +             return 4;
>
>
>Shaopeng TAN,
>
>Good catch. Your solution is the correct one.
>
>Will fix in next post.

I played around with this code a little and I think the logical
expressions are returning wrong values.

On a system that has SNC disabled the function reports both "node_cpus"
and "cache_cpus" equal to 56. In this case snc_ways() returns "2". It is
the same on a system with SNC enabled that reports the previously mentioned
variables to be different by a factor of two (36 and 72).

Is it possible for node_cpus and cache_cpus to not be multiples of each
other? (as in for example cache_cpus being 10 and node_cpus being 21?).
If not I'd suggest using "==" instead of ">=".

If yes then I guess something like this could work? :

+     if (node_cpus >= cache_cpus)
+             return 1;
+     else if (2 * node_cpus >= cache_cpus)
+             return 2;
+     else if (4 * node_cpus >= cache_cpus)
+             return 4;

PS. I did my tests on two Intel Ice Lakes.

-- 
Kind regards
Maciej Wieczór-Retman