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

Phil Auld
2022-09-23T00:38:19Z

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

>
> Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.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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