Re: [PATCH 1/2] cgroup/cpuset: record DL BW alloc CPU for attach rollback

[Chen Ridong]

On 2026/4/17 11:37, Guopeng Zhang wrote:
> cpuset_can_attach() allocates DL bandwidth only when migrating
> deadline tasks to a disjoint CPU mask, but cpuset_cancel_attach()
> rolls back based only on nr_migrate_dl_tasks. This makes the DL
> bandwidth alloc/free paths asymmetric: rollback can call dl_bw_free()
> even when no dl_bw_alloc() was done.
> 
> Rollback also needs to undo the reservation against the same CPU/root
> domain that was charged. Record the CPU used by dl_bw_alloc() and use
> that state in cpuset_cancel_attach(). If no allocation happened,
> dl_bw_cpu stays at -1 and rollback skips dl_bw_free(). If allocation
> did happen, bandwidth is returned to the same CPU/root domain.
> 
> Successful attach paths are unchanged. This only fixes failed attach
> rollback accounting.
> 
> Fixes: 2ef269ef1ac0 ("cgroup/cpuset: Free DL BW in case can_attach() fails")
> Signed-off-by: Guopeng Zhang <zhangguopeng@xxxxxxxxxx>
> ---
>  kernel/cgroup/cpuset-internal.h |  5 +++++
>  kernel/cgroup/cpuset.c          | 13 +++++++++----
>  2 files changed, 14 insertions(+), 4 deletions(-)
> 
> diff --git a/kernel/cgroup/cpuset-internal.h b/kernel/cgroup/cpuset-internal.h
> index fd7d19842ded..bb4e692bea30 100644
> --- a/kernel/cgroup/cpuset-internal.h
> +++ b/kernel/cgroup/cpuset-internal.h
> @@ -168,6 +168,11 @@ struct cpuset {
>  	int nr_deadline_tasks;
>  	int nr_migrate_dl_tasks;
>  	u64 sum_migrate_dl_bw;
> +	/*
> +	 * CPU used for temporary DL bandwidth allocation during attach;
> +	 * -1 if no DL bandwidth was allocated in the current attach.
> +	 */
> +	int dl_bw_cpu;
>  
>  	/* Invalid partition error code, not lock protected */
>  	enum prs_errcode prs_err;
> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> index 1335e437098e..e3a081a07c6d 100644
> --- a/kernel/cgroup/cpuset.c
> +++ b/kernel/cgroup/cpuset.c
> @@ -288,6 +288,7 @@ struct cpuset top_cpuset = {
>  	.flags = BIT(CS_CPU_EXCLUSIVE) |
>  		 BIT(CS_MEM_EXCLUSIVE) | BIT(CS_SCHED_LOAD_BALANCE),
>  	.partition_root_state = PRS_ROOT,
> +	.dl_bw_cpu = -1,
>  };
>  
>  /**
> @@ -579,6 +580,8 @@ static struct cpuset *dup_or_alloc_cpuset(struct cpuset *cs)
>  	if (!trial)
>  		return NULL;
>  
> +	trial->dl_bw_cpu = -1;
> +
>  	/* Setup cpumask pointer array */
>  	cpumask_var_t *pmask[4] = {
>  		&trial->cpus_allowed,
> @@ -2980,6 +2983,7 @@ static void reset_migrate_dl_data(struct cpuset *cs)
>  {
>  	cs->nr_migrate_dl_tasks = 0;
>  	cs->sum_migrate_dl_bw = 0;
> +	cs->dl_bw_cpu = -1;
>  }
>  
>  /* Called by cgroups to determine if a cpuset is usable; cpuset_mutex held */
> @@ -3056,6 +3060,8 @@ static int cpuset_can_attach(struct cgroup_taskset *tset)
>  			reset_migrate_dl_data(cs);
>  			goto out_unlock;
>  		}
> +
> +		cs->dl_bw_cpu = cpu;
>  	}
>  
>  out_success:
> @@ -3080,12 +3086,11 @@ static void cpuset_cancel_attach(struct cgroup_taskset *tset)
>  	mutex_lock(&cpuset_mutex);
>  	dec_attach_in_progress_locked(cs);
>  
> -	if (cs->nr_migrate_dl_tasks) {
> -		int cpu = cpumask_any(cs->effective_cpus);
> +	if (cs->dl_bw_cpu >= 0)
> +		dl_bw_free(cs->dl_bw_cpu, cs->sum_migrate_dl_bw);
>  
> -		dl_bw_free(cpu, cs->sum_migrate_dl_bw);
> +	if (cs->nr_migrate_dl_tasks)
>  		reset_migrate_dl_data(cs);
> -	}
>  
>  	mutex_unlock(&cpuset_mutex);
>  }

Good catch.

Reviewed-by: Chen Ridong <chenridong@xxxxxxxxxxxxxxx>

-- 
Best regards,
Ridong