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

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Message-ID: <87zf8ehyf7.ffs@tglx>
Date: Sat, 22 Nov 2025 16:36:28 +0100
From: Thomas Gleixner <tglx@...utronix.de>
To: Marek Szyprowski <m.szyprowski@...sung.com>, LKML
 <linux-kernel@...r.kernel.org>
Cc: 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>,
 "Paul E. McKenney" <paulmck@...nel.org>, "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 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?

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);