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

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Message-ID: <TYAPR01MB633033C489AAC0E514CBC6688BEEA@TYAPR01MB6330.jpnprd01.prod.outlook.com>
Date:   Thu, 7 Sep 2023 05:21:20 +0000
From:   "Shaopeng Tan (Fujitsu)" <tan.shaopeng@...itsu.com>
To:     'Tony Luck' <tony.luck@...el.com>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Reinette Chatre <reinette.chatre@...el.com>,
        Peter Newman <peternewman@...gle.com>,
        Jonathan Corbet <corbet@....net>,
        Shuah Khan <skhan@...uxfoundation.org>,
        "x86@...nel.org" <x86@...nel.org>
CC:     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

Hi Tony,

> Sub-NUMA Cluster divides CPUs sharing an L3 cache into separate NUMA
> nodes. Systems may support splitting into either two or four nodes.
> 
> When SNC mode is enabled the effective amount of L3 cache available for
> allocation is divided by the number of nodes per L3.
> 
> Detect which SNC mode is active by comparing the number of CPUs that share
> a cache with CPU0, with the number of CPUs on node0.
> 
> This gives some hope of tests passing. But additional test infrastructure
> changes are needed to bind tests to nodes and guarantee memory allocation
> from the local node.
> 
> Reported-by: "Shaopeng Tan (Fujitsu)" <tan.shaopeng@...itsu.com>
> Signed-off-by: Tony Luck <tony.luck@...el.com>
> ---
>  tools/testing/selftests/resctrl/resctrl.h   |  1 +
>  tools/testing/selftests/resctrl/resctrlfs.c | 57
> +++++++++++++++++++++
>  2 files changed, 58 insertions(+)
> 
> diff --git a/tools/testing/selftests/resctrl/resctrl.h
> b/tools/testing/selftests/resctrl/resctrl.h
> index 87e39456dee0..a8b43210b573 100644
> --- a/tools/testing/selftests/resctrl/resctrl.h
> +++ b/tools/testing/selftests/resctrl/resctrl.h
> @@ -13,6 +13,7 @@
>  #include <signal.h>
>  #include <dirent.h>
>  #include <stdbool.h>
> +#include <ctype.h>
>  #include <sys/stat.h>
>  #include <sys/ioctl.h>
>  #include <sys/mount.h>
> diff --git a/tools/testing/selftests/resctrl/resctrlfs.c
> b/tools/testing/selftests/resctrl/resctrlfs.c
> index fb00245dee92..79eecbf9f863 100644
> --- a/tools/testing/selftests/resctrl/resctrlfs.c
> +++ b/tools/testing/selftests/resctrl/resctrlfs.c
> @@ -130,6 +130,61 @@ int get_resource_id(int cpu_no, int *resource_id)
>  	return 0;
>  }
> 
> +/*
> + * Count number of CPUs in a /sys bit map  */ static int
> +count_sys_bitmap_bits(char *name) {
> +	FILE *fp = fopen(name, "r");
> +	int count = 0, c;
> +
> +	if (!fp)
> +		return 0;
> +
> +	while ((c = fgetc(fp)) != EOF) {
> +		if (!isxdigit(c))
> +			continue;
> +		switch (c) {
> +		case 'f':
> +			count++;
> +		case '7': case 'b': case 'd': case 'e':
> +			count++;
> +		case '3': case '5': case '6': case '9': case 'a': case 'c':
> +			count++;
> +		case '1': case '2': case '4': case '8':
> +			count++;
> +		}
> +	}
> +	fclose(fp);
> +
> +	return count;
> +}
> +
> +/*
> + * Detect SNC by compating #CPUs in node0 with #CPUs sharing LLC with
> +CPU0
> + * Try to get this right, even if a few CPUs are offline so that the
> +number
> + * of CPUs in node0 is not exactly half or a quarter of the CPUs
> +sharing the
> + * LLC of CPU0.
> + */
> +static int snc_ways(void)
> +{
> +	int node_cpus, cache_cpus;
> +
> +	node_cpus =
> count_sys_bitmap_bits("/sys/devices/system/node/node0/cpumap");
> +	cache_cpus =
> +count_sys_bitmap_bits("/sys/devices/system/cpu/cpu0/cache/index3/sh
> ared
> +_cpu_map");
> +
> +	if (!node_cpus || !cache_cpus) {
> +		fprintf(stderr, "Warning could not determine Sub-NUMA
> Cluster mode\n");
> +		return 1;
> +	}
> +
> +	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;

Best regards,
Shaopeng TAN