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

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Date:   Fri, 23 Sep 2022 13:03:54 +0300
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     Phil Auld <pauld@...hat.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

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

Thank you!

-- 
With Best Regards,
Andy Shevchenko