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SubjectRe: [PATCH v2 1/1] cpumask: Don't waste memory for sysfs cpulist nodes
 + Petr Štetiar <ynezz@true.cz>,
+ Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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.

1861

>
> 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)

The defs below will be nicer if you make it like this:

#define __CPULIST_FOR_10(x) (((x + 1) / 2 - 0) * 2)
#define __CPULIST_FOR_100(x) __CPULIST_FOR_10(10) + (((x + 1) / 2 - 5) * 3)
#define __CPULIST_FOR_1000(x) __CPULIST_FOR_100(100) + (((x + 1) / 2 - 50) * 4)
...



> +#if NR_CPUS < 1861
> +#define CPULIST_FILE_MAX_BYTES PAGE_SIZE

The comment says:
for more than 1170 and less than 1861 CPUs the page size is preserved.

Which doesn't look correct. Looks like it should be:
for less than 1861 CPUs the page size is preserved.

Or I miss something?

> +#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

I'm OK to take this in replace for Phil's version, but the commit that
introduces CPULIST_FILE_MAX_BYTES is already in mainline: 7ee951acd31a8
("drivers/base: fix userspace break from using bin_attributes for cpumap
and cpulist"). Can you rebase it on top of v6.0-rc6?

Greg, since Andy's version is more precise, I'd like to send a pull
request with it in -rc7. Can you drop Phil's patch so I'll go with
this one?

Thanks,
Yury

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