Re: [PATCH v2 5/5] lib/cpumask: add FORCE_NR_CPUS config option

From: Geert Uytterhoeven <geert@linux-m68k.org>
To: Yury Norov <yury.norov@gmail.com>
Cc: linux-kernel@vger.kernel.org,
	Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
	Rasmus Villemoes <linux@rasmusvillemoes.dk>,
	Andrew Morton <akpm@linux-foundation.org>,
	Stephen Rothwell <sfr@canb.auug.org.au>,
	Peter Zijlstra <peterz@infradead.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	"Paul E . McKenney" <paulmck@kernel.org>,
	Vlastimil Babka <vbabka@suse.cz>,
	Dmitry Vyukov <dvyukov@google.com>,
	Valentin Schneider <vschneid@redhat.com>,
	Sander Vanheule <sander@svanheule.net>,
	Alexey Klimov <klimov.linux@gmail.com>,
	Eric Biggers <ebiggers@google.com>,
	linux-riscv <linux-riscv@lists.infradead.org>
Subject: Re: [PATCH v2 5/5] lib/cpumask: add FORCE_NR_CPUS config option
Date: Tue, 18 Oct 2022 15:15:32 +0200	[thread overview]
Message-ID: <CAMuHMdW2H4egcmv238Q_5LBeu9BE=H1fm=n4vZGCGYzhg2VN1A@mail.gmail.com> (raw)
In-Reply-To: <CAMuHMdUL0WxYjfRDxLWZG7Xu=2xTQkueathvpwWfLpqhG6NkFQ@mail.gmail.com>

Hi Yury,

On Tue, Oct 18, 2022 at 10:21 AM Geert Uytterhoeven
<geert@linux-m68k.org> wrote:
> On Tue, Sep 6, 2022 at 1:10 AM Yury Norov <yury.norov@gmail.com> wrote:
> > The size of cpumasks is hard-limited by compile-time parameter NR_CPUS,
> > but defined at boot-time when kernel parses ACPI/DT tables, and stored in
> > nr_cpu_ids. In many practical cases, number of CPUs for a target is known
> > at compile time, and can be provided with NR_CPUS.
> >
> > In that case, compiler may be instructed to rely on NR_CPUS as on actual
> > number of CPUs, not an upper limit. It allows to optimize many cpumask
> > routines and significantly shrink size of the kernel image.
> >
> > This patch adds FORCE_NR_CPUS option to teach the compiler to rely on
> > NR_CPUS and enable corresponding optimizations.
> >
> > If FORCE_NR_CPUS=y, kernel will not set nr_cpu_ids at boot, but only check
> > that the actual number of possible CPUs is equal to NR_CPUS, and WARN if
> > that doesn't hold.
> >
> > The new option is especially useful in embedded applications because
> > kernel configurations are unique for each SoC, the number of CPUs is
> > constant and known well, and memory limitations are typically harder.
> >
> > For my 4-CPU ARM64 build with NR_CPUS=4, FORCE_NR_CPUS=y saves 46KB:
> >   add/remove: 3/4 grow/shrink: 46/729 up/down: 652/-46952 (-46300)
> >
> > Signed-off-by: Yury Norov <yury.norov@gmail.com>
>
> Thanks for your patch, which is now commit 6f9c07be9d020489
> ("lib/cpumask: add FORCE_NR_CPUS config option") in v6.1-rc1.
>
> FORCE_NR_CPUS is enabled for e.g. an allmodconfig kernel, which I
> believe now makes it unsafe to boot such a kernel on any system that
> does not have exactly CONFIG_NR_CPUS CPU cores?
>
> If my assumption is true, this really needs some protection to prevent
> enabling this option inadvertently, as it is quite common to boot
> allmodconfig kernels for testing.

Moreover, this cannot be used on all systems.  E.g. on Icicle Kit with
Microchip PolarFire SoC, CONFIG_NR_CPUS needs to be larger than 4,
as the system has actually 5 CPU cores (1xE51 and 4xU54), but Linux
runs only on 4 of them.  So you cannot use FORCE_NR_CPUS=y.

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds