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Date:   Thu, 2 Nov 2023 10:48:01 -0700
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Subject: Re: [PATCH 06/24] selftests/resctrl: Exclude shareable bits from
 schemata in CAT test
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To:     =?UTF-8?Q?Ilpo_J=C3=A4rvinen?= <ilpo.jarvinen@linux.intel.com>,
        <linux-kselftest@vger.kernel.org>, Shuah Khan <shuah@kernel.org>,
        "Shaopeng Tan" <tan.shaopeng@jp.fujitsu.com>,
        =?UTF-8?Q?Maciej_Wiecz=C3=B3r-Retman?= 
        <maciej.wieczor-retman@intel.com>,
        Fenghua Yu <fenghua.yu@intel.com>
CC:     <linux-kernel@vger.kernel.org>
References: <20231024092634.7122-1-ilpo.jarvinen@linux.intel.com>
 <20231024092634.7122-7-ilpo.jarvinen@linux.intel.com>
From:   Reinette Chatre <reinette.chatre@intel.com>
In-Reply-To: <20231024092634.7122-7-ilpo.jarvinen@linux.intel.com>
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Hi Ilpo,

On 10/24/2023 2:26 AM, Ilpo Järvinen wrote:
> CAT test doesn't take shareable bits into account, i.e., the test might
> be sharing cache with some devices (e.g., graphics).
> 
> Introduce get_mask_no_shareable() and use it to provision an
> environment for CAT test where the allocated LLC is isolated better.
> Excluding shareable_bits may create hole(s) into the cbm_mask, thus add
> a new helper count_contiguous_bits() to find the longest contiguous set
> of CBM bits.
> 
> create_bit_mask() is needed by an upcoming CAT test rewrite so make it
> available in resctrl.h right away.
> 
> Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
> ---
>  tools/testing/selftests/resctrl/cat_test.c  | 12 ++-
>  tools/testing/selftests/resctrl/resctrl.h   |  3 +
>  tools/testing/selftests/resctrl/resctrlfs.c | 84 +++++++++++++++++++++
>  3 files changed, 95 insertions(+), 4 deletions(-)
> 
> diff --git a/tools/testing/selftests/resctrl/cat_test.c b/tools/testing/selftests/resctrl/cat_test.c
> index 80861c362a53..e5861e7cba7e 100644
> --- a/tools/testing/selftests/resctrl/cat_test.c
> +++ b/tools/testing/selftests/resctrl/cat_test.c
> @@ -92,13 +92,17 @@ int cat_perf_miss_val(int cpu_no, int n, char *cache_type)
>  	unsigned long l_mask, l_mask_1;
>  	int ret, pipefd[2], sibling_cpu_no;
>  	unsigned long cache_total_size = 0;
> -	unsigned long long_mask;
> +	unsigned long full_cache_mask, long_mask;

Please do maintain reverse fir order.

>  	int count_of_bits;
>  	char pipe_message;
>  	size_t span;
>  
>  	/* Get default cbm mask for L3/L2 cache */
> -	ret = get_cbm_mask(cache_type, &long_mask);
> +	ret = get_cbm_mask(cache_type, &full_cache_mask);
> +	if (ret)
> +		return ret;
> +	/* Get the exclusive portion of the cache */
> +	ret = get_mask_no_shareable(cache_type, &long_mask);
>  	if (ret)
>  		return ret;
>  
> +
> +/*
> + * count_contiguous_bits - Returns the longest train of bits in a bit mask
> + * @val		A bit mask
> + * @start	The location of the least-significant bit of the longest train
> + *
> + * Return:	The length of the contiguous bits in the longest train of bits
> + */
> +static unsigned int count_contiguous_bits(unsigned long val, unsigned int *start)
> +{
> +	unsigned long last_val;
> +	int count = 0;

could count be unsigned int?

> +
> +	while (val) {
> +		last_val = val;
> +		val &= (val >> 1);
> +		count++;
> +	}
> +
> +	if (start) {
> +		if (count)
> +			*start = ffsl(last_val) - 1;
> +		else
> +			*start = 0;
> +	}
> +
> +	return count;
> +}
> +
>  /*
>   * get_cbm_mask - Get cbm bit mask
>   * @cache_type:		Cache level L2/L3
> @@ -253,6 +291,52 @@ int get_cbm_mask(const char *cache_type, unsigned long *mask)
>  	return 0;
>  }
>  
> +/*
> + * get_shareable_mask - Get shareable mask from shareable_bits for given cache
> + * @cache_type:		Cache level L2/L3
> + * @shareable_mask:	shareable mask returned as unsigned long

A nitpick but please be consistent in starting sentences with capital letter.

> + *
> + * Return: = 0 on success, < 0 on failure.
> + */
> +int get_shareable_mask(const char *cache_type, unsigned long *shareable_mask)
> +{
> +	char mask_path[1024];
> +
> +	if (!cache_type)
> +		return -1;

Same question as earlier about error checking.

> +
> +	snprintf(mask_path, sizeof(mask_path), "%s/%s/shareable_bits",
> +		 INFO_PATH, cache_type);
> +
> +	return get_bit_mask(mask_path, shareable_mask);
> +}
> +
> +/*
> + * get_mask_no_shareable - Get CBM mask without shareable_bits for given cache
> + * @cache_type:		Cache level L2/L3
> + * @mask:		mask returned as unsigned long

Apart from comment about capital letter this comment does not seem to
reveal more about what code does.

> + *
> + * Return: = 0 on success, < 0 on failure.
> + */
> +int get_mask_no_shareable(const char *cache_type, unsigned long *mask)
> +{
> +	unsigned long full_mask, shareable_mask;
> +	unsigned int start, len;
> +
> +	if (get_cbm_mask(cache_type, &full_mask) < 0)
> +		return -1;
> +	if (get_shareable_mask(cache_type, &shareable_mask) < 0)
> +		return -1;
> +
> +	len = count_contiguous_bits(full_mask & ~shareable_mask, &start);
> +	if (!len)
> +		return -1;
> +
> +	*mask = create_bit_mask(start, len);
> +
> +	return 0;
> +}

Nicely done, thanks.

> +
>  /*
>   * get_core_sibling - Get sibling core id from the same socket for given CPU
>   * @cpu_no:	CPU number


Reinette