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 18:45:27 -0700
From:   Yury Norov <yury.norov@...il.com>
To:     Phil Auld <pauld@...hat.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Petr Štetiar <ynezz@...e.cz>
Cc:     Andy Shevchenko <andriy.shevchenko@...ux.intel.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 5:38 PM Phil Auld <pauld@...hat.com> wrote:
>
> 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>

By the way, why don't we use nr_cpu_ids? It's set at boot-time, or even
at compile-time in some cases and never changed at runtime.

nr_cpu_ids is set very early, when ACPI tables are parsed. I don't think
it's possible for a userspace to observe it uninitialized. Am I wrong?