linux-kernel - Re: [PATCH v2 1/1] cpumask: Don't waste memory for sysfs cpulist nodes

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Date:   Thu, 22 Sep 2022 20:38:06 -0400
From:   Phil Auld <pauld@...hat.com>
To:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Cc:     Yury Norov <yury.norov@...il.com>, linux-kernel@...r.kernel.org,
        Rasmus Villemoes <linux@...musvillemoes.dk>
Subject: Re: [PATCH v2 1/1] cpumask: Don't waste memory for sysfs cpulist
 nodes

Hi Andy,

On Thu, Sep 22, 2022 at 10:49:54PM +0300 Andy Shevchenko wrote:
> Currently the approximation is used which wastes the more memory
> the more CPUs are present on the system. Proposed change calculates
> the exact maximum needed in the worst case:
> 
>   NR_CPUS	old		new
>   -------	---		---
>   1 .. 1170	4096		4096
>   1171 .. 1860	4098 ..	6510	4096
>   ...		...		...
>   2*4096	28672		19925
>   4*4096	57344		43597
>   8*4096	114688		92749
>   16*4096	229376		191053
>   32*4096	458752		403197
>   64*4096	917504		861949
>   128*4096	1835008		1779453
>   256*4096	3670016		3670016
> 
> Under the hood the reccurent formula is being used:
>   (5 - 0) * 2 +
>     (50 - 5) * 3 +
>       (500 - 50) * 4 +
>         (5000 - 500) * 5 +
>           ...
>             (X[i] - X[i-1]) * i
> 
> which allows to count the exact maximum length in the worst case,
> i.e. when each second CPU is being listed. For backward compatibility
> for more than 1170 and less than 1861 CPUs the page size is preserved.
> 
> For less than 1171 and more than 1 million CPUs the old is being used.

The memory is not really wasted since it's probably temporary in userspace
and in the kernel it _is_ temporary and is only the length of the kasprintf
string, which is most of the time much less.

But that said, it is more accurate than the previous estimate.

I was wondering if you were going to try to come up with a suitable
compile time macro :)

I tested 2, 8192 and 16k since the kernel does not want to build for other
reasons with NR_CPUS at 32k.

Reviewed-by: Phil Auld <pauld@...hat.com>
Tested-by: Phil Auld <pauld@...hat.com>


> 
> Signed-off-by: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
> ---
> v2: described better the advantage for 1171..1860 CPUs cases
>  include/linux/cpumask.h | 48 +++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 48 insertions(+)
> 
> diff --git a/include/linux/cpumask.h b/include/linux/cpumask.h
> index 1b442fb2001f..12cf0905ca74 100644
> --- a/include/linux/cpumask.h
> +++ b/include/linux/cpumask.h
> @@ -1122,6 +1122,21 @@ cpumap_print_list_to_buf(char *buf, const struct cpumask *mask,
>   *
>   * for cpumap NR_CPUS * 9/32 - 1 should be an exact length.
>   *
> + * for cpulist the reccurent formula is being used:
> + *   (5 - 0) * 2 +
> + *     (50 - 5) * 3 +
> + *       (500 - 50) * 4 +
> + *         (5000 - 500) * 5 +
> + *           ...
> + *             (X[i] - X[i-1]) * i
> + *
> + * which allows to count the exact maximum length in the worst case,
> + * i.e. when each second CPU is being listed. For backward compatibility
> + * for more than 1170 and less than 1861 CPUs the page size is preserved.
> + *
> + * For less than 1171 and more than 1 million CPUs the old is being used
> + * as described below:
> + *
>   * For cpulist 7 is (ceil(log10(NR_CPUS)) + 1) allowing for NR_CPUS to be up
>   * to 2 orders of magnitude larger than 8192. And then we divide by 2 to
>   * cover a worst-case of every other cpu being on one of two nodes for a
> @@ -1132,6 +1147,39 @@ cpumap_print_list_to_buf(char *buf, const struct cpumask *mask,
>   */
>  #define CPUMAP_FILE_MAX_BYTES  (((NR_CPUS * 9)/32 > PAGE_SIZE) \
>  					? (NR_CPUS * 9)/32 - 1 : PAGE_SIZE)
> +
> +#define __CPULIST_FOR_10(x)		(((x + 1) / 2 - 0)     * 2)
> +#define __CPULIST_FOR_100(x)		(((x + 1) / 2 - 5)     * 3)
> +#define __CPULIST_FOR_1000(x)		(((x + 1) / 2 - 50)    * 4)
> +#define __CPULIST_FOR_10000(x)		(((x + 1) / 2 - 500)   * 5)
> +#define __CPULIST_FOR_100000(x)		(((x + 1) / 2 - 5000)  * 6)
> +#define __CPULIST_FOR_1000000(x)	(((x + 1) / 2 - 50000) * 7)
> +
> +#if NR_CPUS < 1861
> +#define CPULIST_FILE_MAX_BYTES	PAGE_SIZE
> +#elif NR_CPUS < 10000
> +#define CPULIST_FILE_MAX_BYTES			\
> +	 (__CPULIST_FOR_10(10) +		\
> +	  __CPULIST_FOR_100(100) +		\
> +	  __CPULIST_FOR_1000(1000) +		\
> +	  __CPULIST_FOR_10000(NR_CPUS))
> +#elif NR_CPUS < 100000
> +#define CPULIST_FILE_MAX_BYTES			\
> +	 (__CPULIST_FOR_10(10) +		\
> +	  __CPULIST_FOR_100(100) +		\
> +	  __CPULIST_FOR_1000(1000) +		\
> +	  __CPULIST_FOR_10000(10000) +		\
> +	  __CPULIST_FOR_100000(NR_CPUS))
> +#elif NR_CPUS < 1000000
> +#define CPULIST_FILE_MAX_BYTES			\
> +	 (__CPULIST_FOR_10(10) +		\
> +	  __CPULIST_FOR_100(100) +		\
> +	  __CPULIST_FOR_1000(1000) +		\
> +	  __CPULIST_FOR_10000(10000) +		\
> +	  __CPULIST_FOR_100000(100000) +	\
> +	  __CPULIST_FOR_1000000(NR_CPUS))
> +#else
>  #define CPULIST_FILE_MAX_BYTES  (((NR_CPUS * 7)/2 > PAGE_SIZE) ? (NR_CPUS * 7)/2 : PAGE_SIZE)
> +#endif
>  
>  #endif /* __LINUX_CPUMASK_H */
> -- 
> 2.35.1
> 


Cheers,
Phil


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