Message-ID: <a5e3294b-4d37-4e98-9442-e35ca1949c17@redhat.com>
Date: Wed, 5 Nov 2025 14:33:57 -0500
From: Waiman Long <llong@...hat.com>
To: Frederic Weisbecker <frederic@...nel.org>, Waiman Long <llong@...hat.com>
Cc: LKML <linux-kernel@...r.kernel.org>, Michal Koutný
 <mkoutny@...e.com>, Andrew Morton <akpm@...ux-foundation.org>,
 Bjorn Helgaas <bhelgaas@...gle.com>,
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Subject: Re: [PATCH 13/33] cpuset: Update HK_TYPE_DOMAIN cpumask from cpuset

On 11/5/25 10:42 AM, Frederic Weisbecker wrote:
> Le Tue, Oct 21, 2025 at 12:10:16AM -0400, Waiman Long a écrit :
>> On 10/13/25 4:31 PM, Frederic Weisbecker wrote:
>>> Until now, HK_TYPE_DOMAIN used to only include boot defined isolated
>>> CPUs passed through isolcpus= boot option. Users interested in also
>>> knowing the runtime defined isolated CPUs through cpuset must use
>>> different APIs: cpuset_cpu_is_isolated(), cpu_is_isolated(), etc...
>>>
>>> There are many drawbacks to that approach:
>>>
>>> 1) Most interested subsystems want to know about all isolated CPUs, not
>>>     just those defined on boot time.
>>>
>>> 2) cpuset_cpu_is_isolated() / cpu_is_isolated() are not synchronized with
>>>     concurrent cpuset changes.
>>>
>>> 3) Further cpuset modifications are not propagated to subsystems
>>>
>>> Solve 1) and 2) and centralize all isolated CPUs within the
>>> HK_TYPE_DOMAIN housekeeping cpumask.
>>>
>>> Subsystems can rely on RCU to synchronize against concurrent changes.
>>>
>>> The propagation mentioned in 3) will be handled in further patches.
>>>
>>> Signed-off-by: Frederic Weisbecker <frederic@...nel.org>
>>> ---
>>>    include/linux/sched/isolation.h |  2 +
>>>    kernel/cgroup/cpuset.c          |  2 +
>>>    kernel/sched/isolation.c        | 75 ++++++++++++++++++++++++++++++---
>>>    kernel/sched/sched.h            |  1 +
>>>    4 files changed, 74 insertions(+), 6 deletions(-)
>>>
>>> diff --git a/include/linux/sched/isolation.h b/include/linux/sched/isolation.h
>>> index da22b038942a..94d5c835121b 100644
>>> --- a/include/linux/sched/isolation.h
>>> +++ b/include/linux/sched/isolation.h
>>> @@ -32,6 +32,7 @@ extern const struct cpumask *housekeeping_cpumask(enum hk_type type);
>>>    extern bool housekeeping_enabled(enum hk_type type);
>>>    extern void housekeeping_affine(struct task_struct *t, enum hk_type type);
>>>    extern bool housekeeping_test_cpu(int cpu, enum hk_type type);
>>> +extern int housekeeping_update(struct cpumask *mask, enum hk_type type);
>>>    extern void __init housekeeping_init(void);
>>>    #else
>>> @@ -59,6 +60,7 @@ static inline bool housekeeping_test_cpu(int cpu, enum hk_type type)
>>>    	return true;
>>>    }
>>> +static inline int housekeeping_update(struct cpumask *mask, enum hk_type type) { return 0; }
>>>    static inline void housekeeping_init(void) { }
>>>    #endif /* CONFIG_CPU_ISOLATION */
>>> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
>>> index aa1ac7bcf2ea..b04a4242f2fa 100644
>>> --- a/kernel/cgroup/cpuset.c
>>> +++ b/kernel/cgroup/cpuset.c
>>> @@ -1403,6 +1403,8 @@ static void update_unbound_workqueue_cpumask(bool isolcpus_updated)
>>>    	ret = workqueue_unbound_exclude_cpumask(isolated_cpus);
>>>    	WARN_ON_ONCE(ret < 0);
>>> +	ret = housekeeping_update(isolated_cpus, HK_TYPE_DOMAIN);
>>> +	WARN_ON_ONCE(ret < 0);
>>>    }
>>>    /**
>>> diff --git a/kernel/sched/isolation.c b/kernel/sched/isolation.c
>>> index b46c20b5437f..95d69c2102f6 100644
>>> --- a/kernel/sched/isolation.c
>>> +++ b/kernel/sched/isolation.c
>>> @@ -29,18 +29,48 @@ static struct housekeeping housekeeping;
>>>    bool housekeeping_enabled(enum hk_type type)
>>>    {
>>> -	return !!(housekeeping.flags & BIT(type));
>>> +	return !!(READ_ONCE(housekeeping.flags) & BIT(type));
>>>    }
>>>    EXPORT_SYMBOL_GPL(housekeeping_enabled);
>>> +static bool housekeeping_dereference_check(enum hk_type type)
>>> +{
>>> +	if (IS_ENABLED(CONFIG_LOCKDEP) && type == HK_TYPE_DOMAIN) {
>>> +		/* Cpuset isn't even writable yet? */
>>> +		if (system_state <= SYSTEM_SCHEDULING)
>>> +			return true;
>>> +
>>> +		/* CPU hotplug write locked, so cpuset partition can't be overwritten */
>>> +		if (IS_ENABLED(CONFIG_HOTPLUG_CPU) && lockdep_is_cpus_write_held())
>>> +			return true;
>>> +
>>> +		/* Cpuset lock held, partitions not writable */
>>> +		if (IS_ENABLED(CONFIG_CPUSETS) && lockdep_is_cpuset_held())
>>> +			return true;
>> I have some doubt about this condition as the cpuset_mutex may be held in
>> the process of making changes to an isolated partition that will impact
>> HK_TYPE_DOMAIN cpumask.
> Indeed and therefore if the current process is holding the cpuset mutex,
> it is guaranteed that no other process will update the housekeeping cpumask
> concurrently.
>
> So the housekeeping mask is guaranteed to be stable, right? Of course
> the current task may be changing it but while it is changing it, it is
> not reading it.

Right. The lockdep check is for the current task, not other tasks that 
holding the lock.

Thanks,
Longman

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