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Message-ID: <58ee9610-174c-46b0-8b2d-71b5734d6f8b@huaweicloud.com>
Date: Sat, 31 Jan 2026 09:49:04 +0800
From: Chen Ridong <chenridong@...weicloud.com>
To: Waiman Long <llong@...hat.com>, Tejun Heo <tj@...nel.org>,
Johannes Weiner <hannes@...xchg.org>, Michal Koutný
<mkoutny@...e.com>, Ingo Molnar <mingo@...hat.com>,
Peter Zijlstra <peterz@...radead.org>, Juri Lelli <juri.lelli@...hat.com>,
Vincent Guittot <vincent.guittot@...aro.org>,
Steven Rostedt <rostedt@...dmis.org>, Ben Segall <bsegall@...gle.com>,
Mel Gorman <mgorman@...e.de>, Valentin Schneider <vschneid@...hat.com>,
Anna-Maria Behnsen <anna-maria@...utronix.de>,
Frederic Weisbecker <frederic@...nel.org>,
Thomas Gleixner <tglx@...utronix.de>, Shuah Khan <shuah@...nel.org>
Cc: cgroups@...r.kernel.org, linux-kernel@...r.kernel.org,
linux-kselftest@...r.kernel.org
Subject: Re: [PATCH/for-next v2 1/2] cgroup/cpuset: Defer
housekeeping_update() call from CPU hotplug to workqueue
On 2026/1/31 9:43, Chen Ridong wrote:
>
>
> On 2026/1/31 9:06, Waiman Long wrote:
>>
>> On 1/30/26 7:47 PM, Chen Ridong wrote:
>>>
>>> On 2026/1/30 23:42, Waiman Long wrote:
>>>> The update_isolation_cpumasks() function can be called either directly
>>>> from regular cpuset control file write with cpuset_full_lock() called
>>>> or via the CPU hotplug path with cpus_write_lock and cpuset_mutex held.
>> Note this statement.
>
> Thank you for reminder.
>
>>>>
>>>> As we are going to enable dynamic update to the nozh_full housekeeping
>>>> cpumask (HK_TYPE_KERNEL_NOISE) soon with the help of CPU hotplug,
>>>> allowing the CPU hotplug path to call into housekeeping_update() directly
>>>> from update_isolation_cpumasks() will likely cause deadlock. So we
>>>> have to defer any call to housekeeping_update() after the CPU hotplug
>>>> operation has finished. This is now done via the workqueue where
>>>> the actual housekeeping_update() call, if needed, will happen after
>>>> cpus_write_lock is released.
>>>>
>>>> We can't use the synchronous task_work API as call from CPU hotplug
>>>> path happen in the per-cpu kthread of the CPU that is being shut down
>>>> or brought up. Because of the asynchronous nature of workqueue, the
>>>> HK_TYPE_DOMAIN housekeeping cpumask will be updated a bit later than the
>>>> "cpuset.cpus.isolated" control file in this case.
>>>>
>>>> Also add a check in test_cpuset_prs.sh and modify some existing
>>>> test cases to confirm that "cpuset.cpus.isolated" and HK_TYPE_DOMAIN
>>>> housekeeping cpumask will both be updated.
>>>>
>>>> Signed-off-by: Waiman Long <longman@...hat.com>
>>>> ---
>>>> kernel/cgroup/cpuset.c | 37 +++++++++++++++++--
>>>> .../selftests/cgroup/test_cpuset_prs.sh | 13 +++++--
>>>> 2 files changed, 44 insertions(+), 6 deletions(-)
>>>>
>>>> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
>>>> index 7b7d12ab1006..0b0eb1df09d5 100644
>>>> --- a/kernel/cgroup/cpuset.c
>>>> +++ b/kernel/cgroup/cpuset.c
>>>> @@ -84,6 +84,9 @@ static cpumask_var_t isolated_cpus;
>>>> */
>>>> static bool isolated_cpus_updating;
>>>> +/* Both cpuset_mutex and cpus_read_locked acquired */
>>>> +static bool cpuset_locked;
>>>> +
>>>> /*
>>>> * A flag to force sched domain rebuild at the end of an operation.
>>>> * It can be set in
>>>> @@ -285,10 +288,12 @@ void cpuset_full_lock(void)
>>>> {
>>>> cpus_read_lock();
>>>> mutex_lock(&cpuset_mutex);
>>>> + cpuset_locked = true;
>>>> }
>>>> void cpuset_full_unlock(void)
>>>> {
>>>> + cpuset_locked = false;
>>>> mutex_unlock(&cpuset_mutex);
>>>> cpus_read_unlock();
>>>> }
>>>> @@ -1285,6 +1290,16 @@ static bool prstate_housekeeping_conflict(int prstate,
>>>> struct cpumask *new_cpus)
>>>> return false;
>>>> }
>>>> +static void isolcpus_workfn(struct work_struct *work)
>>>> +{
>>>> + cpuset_full_lock();
>>>> + if (isolated_cpus_updating) {
>>>> + WARN_ON_ONCE(housekeeping_update(isolated_cpus) < 0);
>>>> + isolated_cpus_updating = false;
>>>> + }
>>>> + cpuset_full_unlock();
>>>> +}
>>>> +
>>>> /*
>>>> * update_isolation_cpumasks - Update external isolation related CPU masks
>>>> *
>>>> @@ -1293,14 +1308,30 @@ static bool prstate_housekeeping_conflict(int
>>>> prstate, struct cpumask *new_cpus)
>>>> */
>>>> static void update_isolation_cpumasks(void)
>>>> {
>>>> - int ret;
>>>> + static DECLARE_WORK(isolcpus_work, isolcpus_workfn);
>>>> if (!isolated_cpus_updating)
>>>> return;
>>>>
>>> Can this happen?
>>>
>>> cpu0 cpu1
>>> [...]
>>>
>>> isolated_cpus_updating = true;
>>> ...
>>> // 'full_lock' is not acquired
>>> update_isolation_cpumasks
>> That is not true. Either cpus_read_lock or cpus_write_lock and cpuset_mutex are
>> held when update_isolation_cpumasks() is called. So there is mutual exclusion.
>
> Eh, we currently assume that it can only be called from existing scenarios, so
> it's okay for now. But I'm concerned that if we later use
> update_isolation_cpumasks without realizing that we need to hold either
> cpus_write_lock or (cpus_read_lock && cpuset_mutex) , we could run into
> concurrency issues. Maybe I'm worrying too much.
>
> And maybe we shuold add 'lockdep_assert_held' inside the update_isolation_cpumasks.
>
I saw in patch 2/2 that isolated_cpus_updating is described as "protected by
cpuset_top_mutex." This could be a bit ambiguous: the caller need to hold either
cpus_read_lock or cpus_write_lock and cpuset_mutex to protect
isolated_cpus_updating.
>>> // exec worker concurrently
>>> isolcpus_workfn
>>> cpuset_full_lock
>>> isolated_cpus_updating = false;
>>> cpuset_full_unlock();
>>> // This returns uncorrectly
>>> if (!isolated_cpus_updating)
>>> return;
>>>
>> Cheers,
>> Longman
>>
>
--
Best regards,
Ridong
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