linux-kernel - Re: [PATCH/for-next v4 4/4] cgroup/cpuset: Eliminate some duplicated rebuild_sched

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Message-ID: <da363ddb-c006-4ff8-a327-5ef75045d3fd@huaweicloud.com>
Date: Mon, 9 Feb 2026 15:53:47 +0800
From: Chen Ridong <chenridong@...weicloud.com>
To: Waiman Long <longman@...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 v4 4/4] cgroup/cpuset: Eliminate some duplicated
 rebuild_sched_domains() calls



On 2026/2/7 4:37, Waiman Long wrote:
> Now that we are going to defer any changes to the HK_TYPE_DOMAIN
> housekeeping cpumasks to either task_work or workqueue
> where rebuild_sched_domains() call will be issued. The current
> rebuild_sched_domains_locked() call near the end of the cpuset critical
> section can be removed in such cases.
> 
> Currently, a boolean force_sd_rebuild flag is used to decide if
> rebuild_sched_domains_locked() call needs to be invoked. To allow
> deferral that like, we change it to a tri-state sd_rebuild enumaration
> type.
> 
> Signed-off-by: Waiman Long <longman@...hat.com>
> ---
>  kernel/cgroup/cpuset.c | 20 ++++++++++++++------
>  1 file changed, 14 insertions(+), 6 deletions(-)
> 
> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> index d26c77a726b2..e224df321e34 100644
> --- a/kernel/cgroup/cpuset.c
> +++ b/kernel/cgroup/cpuset.c
> @@ -173,7 +173,11 @@ static bool		isolcpus_twork_queued;	/* T */
>   * Note that update_relax_domain_level() in cpuset-v1.c can still call
>   * rebuild_sched_domains_locked() directly without using this flag.
>   */
> -static bool force_sd_rebuild;			/* RWCS */
> +static enum {
> +	SD_NO_REBUILD = 0,
> +	SD_REBUILD,
> +	SD_DEFER_REBUILD,
> +} sd_rebuild;					/* RWCS */
>  
>  /*
>   * Partition root states:
> @@ -990,7 +994,7 @@ void rebuild_sched_domains_locked(void)
>  
>  	lockdep_assert_cpus_held();
>  	lockdep_assert_cpuset_lock_held();
> -	force_sd_rebuild = false;
> +	sd_rebuild = SD_NO_REBUILD;
>  
>  	/* Generate domain masks and attrs */
>  	ndoms = generate_sched_domains(&doms, &attr);
> @@ -1377,6 +1381,9 @@ static void update_isolation_cpumasks(void)
>  	else
>  		isolated_cpus_updating = false;
>  

If isolated_hk_cpus is defined, I believe isolated_cpus_updating becomes redundant.

> +	/* Defer rebuild_sched_domains() to task_work or wq */
> +	sd_rebuild = SD_DEFER_REBUILD;
> +

There is a potential issue: we defer all domain rebuilds here, including those
triggered by hotplug events which may change the isolation state.

The problem is that functions like cpuset_cpu_active, which rely on the
scheduler domains being up-to-date—will, also be delayed. Is that okay?

>  	/*
>  	 * This function can be reached either directly from regular cpuset
>  	 * control file write or via CPU hotplug. In the latter case, it is
> @@ -3011,7 +3018,7 @@ static int update_prstate(struct cpuset *cs, int new_prs)
>  	update_partition_sd_lb(cs, old_prs);
>  
>  	notify_partition_change(cs, old_prs);
> -	if (force_sd_rebuild)
> +	if (sd_rebuild == SD_REBUILD)
>  		rebuild_sched_domains_locked();
>  	free_tmpmasks(&tmpmask);
>  	return 0;
> @@ -3288,7 +3295,7 @@ ssize_t cpuset_write_resmask(struct kernfs_open_file *of,
>  	}
>  
>  	free_cpuset(trialcs);
> -	if (force_sd_rebuild)
> +	if (sd_rebuild == SD_REBUILD)
>  		rebuild_sched_domains_locked();
>  out_unlock:
>  	cpuset_full_unlock();
> @@ -3771,7 +3778,8 @@ hotplug_update_tasks(struct cpuset *cs,
>  
>  void cpuset_force_rebuild(void)
>  {
> -	force_sd_rebuild = true;
> +	if (!sd_rebuild)
> +		sd_rebuild = SD_REBUILD;
>  }
>  
>  /**
> @@ -3981,7 +3989,7 @@ static void cpuset_handle_hotplug(void)
>  	}
>  
>  	/* rebuild sched domains if necessary */
> -	if (force_sd_rebuild)
> +	if (sd_rebuild == SD_REBUILD)
>  		rebuild_sched_domains_cpuslocked();
>  
>  	free_tmpmasks(ptmp);

-- 
Best regards,
Ridong