linux-kernel - Re: [PATCH v6 1/2] selftests/resctrl: Adjust effective L3 cache size with SNC enabled

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Message-ID: <d7353315-9183-417e-8501-8beeb4e77085@intel.com>
Date: Tue, 3 Dec 2024 15:26:41 -0800
From: Reinette Chatre <reinette.chatre@...el.com>
To: Maciej Wieczor-Retman <maciej.wieczor-retman@...el.com>,
	<shuah@...nel.org>, <fenghua.yu@...el.com>
CC: <linux-kernel@...r.kernel.org>, <linux-kselftest@...r.kernel.org>,
	<ilpo.jarvinen@...ux.intel.com>, <tony.luck@...el.com>
Subject: Re: [PATCH v6 1/2] selftests/resctrl: Adjust effective L3 cache size
 with SNC enabled

Hi Maciej,

On 12/2/24 3:08 AM, Maciej Wieczor-Retman wrote:

> diff --git a/tools/testing/selftests/resctrl/resctrlfs.c b/tools/testing/selftests/resctrl/resctrlfs.c
> index d38d6dd90be4..50561993d37c 100644
> --- a/tools/testing/selftests/resctrl/resctrlfs.c
> +++ b/tools/testing/selftests/resctrl/resctrlfs.c
> @@ -13,6 +13,8 @@
>  
>  #include "resctrl.h"
>  
> +int snc_unreliable;
> +
>  static int find_resctrl_mount(char *buffer)
>  {
>  	FILE *mounts;
> @@ -156,6 +158,90 @@ int get_domain_id(const char *resource, int cpu_no, int *domain_id)
>  	return 0;
>  }
>  
> +/*
> + * Count number of CPUs in a /sys bitmap
> + */
> +static unsigned 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);
> +

running this through a syntax checker triggers a couple of complaints due to the
missing break statements. I think this can be made more robust by making use of
"fallthrough" and "break". It looks like this can be obtained by including
linux/compiler.h ... but from what I can tell care should be taken to set
the include directory _after_ includink lib.mk so that top_srcdir is set
correctly.

> +	return count;
> +}
> +
> +static bool cpus_offline_empty(void)
> +{
> +	char offline_cpus_str[64];
> +	FILE *fp;
> +
> +	fp = fopen("/sys/devices/system/cpu/offline", "r");

Please check fp before use.

> +	if (fscanf(fp, "%s", offline_cpus_str) < 0) {

This needs something equivalent to 
46058430fc5d ("selftests/resctrl: Protect against array overflow when reading strings")

> +		if (!errno) {
> +			fclose(fp);
> +			return 1;
> +		}
> +		ksft_perror("Could not read /sys/devices/system/cpu/offline");
> +	}
> +
> +	fclose(fp);
> +
> +	return 0;
> +}
> +
> +/*
> + * Detect SNC by comparing #CPUs in node0 with #CPUs sharing LLC with CPU0.
> + * If any CPUs are offline declare the detection as unreliable and skip the
> + * tests.

nit: "and skip the tests" can be dropped since the function need not make
assumption about how callers will use it.

> + */
> +int snc_nodes_per_l3_cache(void)
> +{
> +	int node_cpus, cache_cpus;
> +	static int snc_mode;
> +
> +	if (!snc_mode) {
> +		snc_mode = 1;
> +		if (!cpus_offline_empty()) {
> +			ksft_print_msg("Runtime SNC detection unreliable due to offline CPUs.\n");
> +			ksft_print_msg("Setting SNC mode to disabled.\n");
> +			snc_unreliable = 1;
> +			return snc_mode;
> +		}
> +		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/shared_cpu_map");
> +
> +		if (!node_cpus || !cache_cpus) {
> +			ksft_print_msg("Could not determine Sub-NUMA Cluster mode.\n");
> +			snc_unreliable = 1;
> +			return snc_mode;
> +		}
> +		snc_mode = cache_cpus / node_cpus;
> +
> +		if (snc_mode > 1)
> +			ksft_print_msg("SNC-%d mode discovered.\n", snc_mode);
> +	}
> +
> +	return snc_mode;
> +}
> +
>  /*
>   * get_cache_size - Get cache size for a specified CPU
>   * @cpu_no:	CPU number
> @@ -211,6 +297,17 @@ int get_cache_size(int cpu_no, const char *cache_type, unsigned long *cache_size
>  			break;
>  	}
>  
> +	/*
> +	 * The amount of cache represented by each bit in the masks
> +	 * in the schemata file is reduced by a factor equal to SNC
> +	 * nodes per L3 cache.
> +	 * E.g. on a SNC-2 system with a 100MB L3 cache a test that
> +	 * allocates memory from its local SNC node (default behavior
> +	 * without using libnuma) will only see 50 MB llc_occupancy
> +	 * with a fully populated L3 mask in the schemata file.
> +	 */
> +	if (cache_num == 3)
> +		*cache_size /= snc_nodes_per_l3_cache();
>  	return 0;
>  }
>  

Reinette