linux-kernel - Re: [patch V5 09/20] cpumask: Cache num_possible

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Message-ID: <74f47d05-e7bd-4b6e-8e2f-9558c20422ed@paulmck-laptop>
Date: Sat, 22 Nov 2025 11:09:49 -0800
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Marek Szyprowski <m.szyprowski@...sung.com>
Cc: Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Gabriele Monaco <gmonaco@...hat.com>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Michael Jeanson <mjeanson@...icios.com>,
	Jens Axboe <axboe@...nel.dk>,
	"Gautham R. Shenoy" <gautham.shenoy@....com>,
	Florian Weimer <fweimer@...hat.com>,
	Tim Chen <tim.c.chen@...el.com>, Yury Norov <yury.norov@...il.com>,
	Shrikanth Hegde <sshegde@...ux.ibm.com>,
	Nathan Chancellor <nathan@...nel.org>
Subject: Re: [patch V5 09/20] cpumask: Cache num_possible_cpus()

On Sat, Nov 22, 2025 at 05:24:22PM +0100, Marek Szyprowski wrote:
> 
> On 22.11.2025 16:36, Thomas Gleixner wrote:
> > On Fri, Nov 21 2025 at 23:56, Marek Szyprowski wrote:
> >> Reverting it on top of linux-next fixes the issue. Let me know how can I
> >> help debugging it.
> > Can you test the fix below please?
> 
> This fixes the observed issue. Thanks!
> 
> Tested-by: Marek Szyprowski <m.szyprowski@...sung.com>

And I clearly should have read ahead, but thank you for the quick fix!

Tested-by: Paul E. McKenney <paulmck@...nel.org>

> > Thanks,
> >
> >          tglx
> > ---
> > Subject: cpu: Initialize __num_possible_cpus correctly
> > From: Thomas Gleixner <tglx@...utronix.de>
> > Date: Sat, 22 Nov 2025 16:19:18 +0100
> >
> > The variable to cache the number of possible CPUs is initialized to NR_CPUS
> > at build time, but that's only correct when cpu_possible_mask is
> > initialized with CPU_BITS_ALL. That's only the case on PARISC.
> >
> > On x86 and some other architectures this does not matter because they
> > initialize cpu_possible_mask via init_cpu_possible() which does a proper
> > weight calculation. Though on architectures which do not, this results
> > in a completely wrong cached value 'NR_CPUS + actual possible CPUs'.
> >
> > Initialize it correctly to 0 when CONFIG_INIT_ALL_POSSIBLE=n and move the
> > NR_CPUS initialization into the PARISC specific section.
> >
> > Fixes: d0f23ccf6ba9 ("cpumask: Cache num_possible_cpus()")
> > Reported-by: Marek Szyprowski <m.szyprowski@...sung.com>
> > Reported-by: Nathan Chancellor <nathan@...nel.org>
> > Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
> > Closes: https://lore.kernel.org/all/89c7106e-a431-443a-9527-3d5fbce77fe1@samsung.com
> > Closes: https://lore.kernel.org/all/20251122002755.GA2682494@ax162
> > ---
> >   kernel/cpu.c |    6 +++---
> >   1 file changed, 3 insertions(+), 3 deletions(-)
> >
> > --- a/kernel/cpu.c
> > +++ b/kernel/cpu.c
> > @@ -3085,10 +3085,13 @@ EXPORT_SYMBOL(cpu_all_bits);
> >   #ifdef CONFIG_INIT_ALL_POSSIBLE
> >   struct cpumask __cpu_possible_mask __ro_after_init
> >   	= {CPU_BITS_ALL};
> > +unsigned int __num_possible_cpus __ro_after_init = NR_CPUS;
> >   #else
> >   struct cpumask __cpu_possible_mask __ro_after_init;
> > +unsigned int __num_possible_cpus __ro_after_init;
> >   #endif
> >   EXPORT_SYMBOL(__cpu_possible_mask);
> > +EXPORT_SYMBOL(__num_possible_cpus);
> >   
> >   struct cpumask __cpu_online_mask __read_mostly;
> >   EXPORT_SYMBOL(__cpu_online_mask);
> > @@ -3108,9 +3111,6 @@ EXPORT_SYMBOL(__cpu_dying_mask);
> >   atomic_t __num_online_cpus __read_mostly;
> >   EXPORT_SYMBOL(__num_online_cpus);
> >   
> > -unsigned int __num_possible_cpus __ro_after_init = NR_CPUS;
> > -EXPORT_SYMBOL(__num_possible_cpus);
> > -
> >   void init_cpu_present(const struct cpumask *src)
> >   {
> >   	cpumask_copy(&__cpu_present_mask, src);
> >
> Best regards
> -- 
> Marek Szyprowski, PhD
> Samsung R&D Institute Poland
>