netdev - Re: [PATCH 13/33] cpuset: Update HK_TYPE

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Message-ID: <ea2d3e0e-b1ee-4b58-a93a-b9d127258e75@redhat.com>
Date: Tue, 21 Oct 2025 09:39:10 -0400
From: Waiman Long <llong@...hat.com>
To: Frederic Weisbecker <frederic@...nel.org>,
 LKML <linux-kernel@...r.kernel.org>
Cc: Michal Koutný <mkoutny@...e.com>,
 Andrew Morton <akpm@...ux-foundation.org>,
 Bjorn Helgaas <bhelgaas@...gle.com>,
 Catalin Marinas <catalin.marinas@....com>, Danilo Krummrich
 <dakr@...nel.org>, "David S . Miller" <davem@...emloft.net>,
 Eric Dumazet <edumazet@...gle.com>, Gabriele Monaco <gmonaco@...hat.com>,
 Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
 Ingo Molnar <mingo@...hat.com>, Jakub Kicinski <kuba@...nel.org>,
 Jens Axboe <axboe@...nel.dk>, Johannes Weiner <hannes@...xchg.org>,
 Lai Jiangshan <jiangshanlai@...il.com>,
 Marco Crivellari <marco.crivellari@...e.com>, Michal Hocko
 <mhocko@...e.com>, Muchun Song <muchun.song@...ux.dev>,
 Paolo Abeni <pabeni@...hat.com>, Peter Zijlstra <peterz@...radead.org>,
 Phil Auld <pauld@...hat.com>, "Rafael J . Wysocki" <rafael@...nel.org>,
 Roman Gushchin <roman.gushchin@...ux.dev>,
 Shakeel Butt <shakeel.butt@...ux.dev>, Simon Horman <horms@...nel.org>,
 Tejun Heo <tj@...nel.org>, Thomas Gleixner <tglx@...utronix.de>,
 Vlastimil Babka <vbabka@...e.cz>, Will Deacon <will@...nel.org>,
 cgroups@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
 linux-block@...r.kernel.org, linux-mm@...ck.org, linux-pci@...r.kernel.org,
 netdev@...r.kernel.org
Subject: Re: [PATCH 13/33] cpuset: Update HK_TYPE_DOMAIN cpumask from cpuset

On 10/13/25 4:31 PM, Frederic Weisbecker wrote:
> @@ -80,12 +110,45 @@ EXPORT_SYMBOL_GPL(housekeeping_affine);
>   
>   bool housekeeping_test_cpu(int cpu, enum hk_type type)
>   {
> -	if (housekeeping.flags & BIT(type))
> +	if (READ_ONCE(housekeeping.flags) & BIT(type))
>   		return cpumask_test_cpu(cpu, housekeeping_cpumask(type));
>   	return true;
>   }
>   EXPORT_SYMBOL_GPL(housekeeping_test_cpu);
>   
> +int housekeeping_update(struct cpumask *mask, enum hk_type type)
> +{
> +	struct cpumask *trial, *old = NULL;
> +
> +	if (type != HK_TYPE_DOMAIN)
> +		return -ENOTSUPP;
> +
> +	trial = kmalloc(sizeof(*trial), GFP_KERNEL);
Should you use cpumask_size() instead of sizeof(*trial) as the latter 
can be much bigger?
> +	if (!trial)
> +		return -ENOMEM;
> +
> +	cpumask_andnot(trial, housekeeping_cpumask(HK_TYPE_DOMAIN_BOOT), mask);
> +	if (!cpumask_intersects(trial, cpu_online_mask)) {
> +		kfree(trial);
> +		return -EINVAL;
> +	}
> +
> +	if (!housekeeping.flags)
> +		static_branch_enable(&housekeeping_overridden);
> +
> +	if (!(housekeeping.flags & BIT(type)))
> +		old = housekeeping_cpumask_dereference(type);
> +	else
> +		WRITE_ONCE(housekeeping.flags, housekeeping.flags | BIT(type));
> +	rcu_assign_pointer(housekeeping.cpumasks[type], trial);
> +
> +	synchronize_rcu();
> +
> +	kfree(old);

If "isolcpus" boot command line option is set, old can be a pointer to 
the boot time memblock area which isn't a pointer that can be handled by 
the slab allocator AFAIU. I don't know the exact consequence, but it may 
not be good. One possible solution I can think of is to make 
HK_TYPE_DOMAIN and HK_TYPE_DOMAIN_ROOT point to the same memblock 
pointer and don't pass the old HK_TYPE_DOMAIN pointer to kfree() if it 
matches HK_TYPE_DOMAIN_BOOT one. Alternatively, we can just set the 
HK_TYPE_DOMAIN_BOOT pointer at boot and make HK_TYPE_DOMAIN falls back 
to HK_TYPE_DOMAIN_BOOT if not set.

Cheers,
Longman