linux-kernel - Re: [PATCH 13/33] sched/isolation: Convert housekeeping cpumasks to rcu pointers

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Message-ID: <18ee9089-8a08-44ed-8761-7c9db765cd4e@redhat.com>
Date: Sun, 11 Jan 2026 21:45:36 -0500
From: Waiman Long <llong@...hat.com>
To: Simon Horman <horms@...nel.org>, Frederic Weisbecker <frederic@...nel.org>
Cc: LKML <linux-kernel@...r.kernel.org>, Michal Koutný
 <mkoutny@...e.com>, Andrew Morton <akpm@...ux-foundation.org>,
 Bjorn Helgaas <bhelgaas@...gle.com>,
 Catalin Marinas <catalin.marinas@....com>,
 Chen Ridong <chenridong@...wei.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>, 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] sched/isolation: Convert housekeeping cpumasks to
 rcu pointers

On 1/7/26 6:56 AM, Simon Horman wrote:
> On Thu, Jan 01, 2026 at 11:13:38PM +0100, Frederic Weisbecker wrote:
>> HK_TYPE_DOMAIN's cpumask will soon be made modifiable by cpuset.
>> A synchronization mechanism is then needed to synchronize the updates
>> with the housekeeping cpumask readers.
>>
>> Turn the housekeeping cpumasks into RCU pointers. Once a housekeeping
>> cpumask will be modified, the update side will wait for an RCU grace
>> period and propagate the change to interested subsystem when deemed
>> necessary.
>>
>> Signed-off-by: Frederic Weisbecker <frederic@...nel.org>
>> ---
>>   kernel/sched/isolation.c | 58 +++++++++++++++++++++++++---------------
>>   kernel/sched/sched.h     |  1 +
>>   2 files changed, 37 insertions(+), 22 deletions(-)
>>
>> diff --git a/kernel/sched/isolation.c b/kernel/sched/isolation.c
>> index 11a623fa6320..83be49ec2b06 100644
>> --- a/kernel/sched/isolation.c
>> +++ b/kernel/sched/isolation.c
>> @@ -21,7 +21,7 @@ DEFINE_STATIC_KEY_FALSE(housekeeping_overridden);
>>   EXPORT_SYMBOL_GPL(housekeeping_overridden);
>>   
>>   struct housekeeping {
>> -	cpumask_var_t cpumasks[HK_TYPE_MAX];
>> +	struct cpumask __rcu *cpumasks[HK_TYPE_MAX];
>>   	unsigned long flags;
>>   };
>>   
>> @@ -33,17 +33,28 @@ bool housekeeping_enabled(enum hk_type type)
>>   }
>>   EXPORT_SYMBOL_GPL(housekeeping_enabled);
>>   
>> +const struct cpumask *housekeeping_cpumask(enum hk_type type)
>> +{
>> +	if (static_branch_unlikely(&housekeeping_overridden)) {
>> +		if (housekeeping.flags & BIT(type)) {
>> +			return rcu_dereference_check(housekeeping.cpumasks[type], 1);
>> +		}
>> +	}
>> +	return cpu_possible_mask;
>> +}
>> +EXPORT_SYMBOL_GPL(housekeeping_cpumask);
>> +
> Hi Frederic,
>
> I think this patch should also update the access to housekeeping.cpumasks
> in housekeeping_setup(), on line 200, to use housekeeping_cpumask().
>
> As is, sparse flags __rcu a annotation miss match there.
>
>    kernel/sched/isolation.c:200:80: warning: incorrect type in argument 3 (different address spaces)
>    kernel/sched/isolation.c:200:80:    expected struct cpumask const *srcp3
>    kernel/sched/isolation.c:200:80:    got struct cpumask [noderef] __rcu *
>
> ...
>
The direct housekeeping.cpumasks[type] reference is in the newly merged 
check after Federic's initial patch series.

                 iter_flags = housekeeping.flags & (HK_FLAG_KERNEL_NOISE 
| HK_FLAG_DOMAIN);
                 type = find_first_bit(&iter_flags, HK_TYPE_MAX);
                 /*
                  * Pass the check if none of these flags were 
previously set or
                  * are not in the current selection.
                  */
                 iter_flags = flags & (HK_FLAG_KERNEL_NOISE | 
HK_FLAG_DOMAIN);
                 first_cpu = (type == HK_TYPE_MAX || !iter_flags) ? 0 :
cpumask_first_and_and(cpu_present_mask,
                                     housekeeping_staging, 
housekeeping.cpumasks[type]);

Maybe that is why it is missed.

Cheers,
Longman