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

Andy Shevchenko
2022-09-23T10:04:10Z

On Thu, Sep 22, 2022 at 08:38:06PM -0400, Phil Auld wrote:
> 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>

Thank you!

--
With Best Regards,
Andy Shevchenko