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

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Date:   Tue, 19 Sep 2023 14:36:06 +0000
From:   "Luck, Tony" <tony.luck@...el.com>
To:     "Wieczor-Retman, Maciej" <maciej.wieczor-retman@...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 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 ">=".

Some CPUs may be offline when the test is run. E.g. with one CPU offline on SNC
node 0, you'd see node_cpus = 35 and cache_cpus = 71. But with one CPU offline
on node 1, you'd have node_cpus = 36, cache_cpus = 71.



> 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;

This returns "4" for the 36 71 case. But should still be "2".

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

Perhaps easier to play with the algorithm in user code?

#include <stdio.h>
#include <stdlib.h>

static int snc(int node_cpus, int cache_cpus)
{
     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;
     return -1;
}

int main(int argc, char **argv)
{
        printf("%d\n", snc(atoi(argv[1]), atoi(argv[2])));

        return 0;
}

N.B. it's probably not possible to handle the case where somebody took ALL the CPUs in SNC
node 1 offline (or SNC nodes 1,2,3 for the SNC 4 case).

I think it reasonable that the code handle some simple "small number of CPUs offline" cases.
But don't worry too much about cases where the user has done something extreme.

-Tony