Re: [PATCH 2/2] cpuset: Call set_cpus_allowed_ptr() with appropriate mask for task

[Waiman Long <longman@redhat.com>]

On 1/31/23 17:17, Will Deacon wrote:
> set_cpus_allowed_ptr() will fail with -EINVAL if the requested
> affinity mask is not a subset of the task_cpu_possible_mask() for the
> task being updated. Consequently, on a heterogeneous system with cpusets
> spanning the different CPU types, updates to the cgroup hierarchy can
> silently fail to update task affinities when the effective affinity
> mask for the cpuset is expanded.
>
> For example, consider an arm64 system with 4 CPUs, where CPUs 2-3 are
> the only cores capable of executing 32-bit tasks. Attaching a 32-bit
> task to a cpuset containing CPUs 0-2 will correctly affine the task to
> CPU 2. Extending the cpuset to CPUs 0-3, however, will fail to extend
> the affinity mask of the 32-bit task because update_tasks_cpumask() will
> pass the full 0-3 mask to set_cpus_allowed_ptr().
>
> Extend update_tasks_cpumask() to take a temporary 'cpumask' paramater
> and use it to mask the 'effective_cpus' mask with the possible mask for
> each task being updated.
>
> Fixes: 431c69fac05b ("cpuset: Honour task_cpu_possible_mask() in guarantee_online_cpus()")
> Signed-off-by: Will Deacon <will@kernel.org>
> ---
>
> Note: We wondered whether it was worth calling guarantee_online_cpus()
> if the cpumask_and() returns 0 in update_tasks_cpumask(), but given that
> this path is only called when the effective mask changes, it didn't
> seem appropriate. Ultimately, if you have 32-bit tasks attached to a
> cpuset containing only 64-bit cpus, then the affinity is going to be
> forced.
>
>   kernel/cgroup/cpuset.c | 18 +++++++++++-------
>   1 file changed, 11 insertions(+), 7 deletions(-)
>
> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> index 8552cc2c586a..f15fb0426707 100644
> --- a/kernel/cgroup/cpuset.c
> +++ b/kernel/cgroup/cpuset.c
> @@ -1205,12 +1205,13 @@ void rebuild_sched_domains(void)
>   /**
>    * update_tasks_cpumask - Update the cpumasks of tasks in the cpuset.
>    * @cs: the cpuset in which each task's cpus_allowed mask needs to be changed
> + * @new_cpus: the temp variable for the new effective_cpus mask
>    *
>    * Iterate through each task of @cs updating its cpus_allowed to the
>    * effective cpuset's.  As this function is called with cpuset_rwsem held,
>    * cpuset membership stays stable.
>    */
> -static void update_tasks_cpumask(struct cpuset *cs)
> +static void update_tasks_cpumask(struct cpuset *cs, struct cpumask *new_cpus)
>   {
>   	struct css_task_iter it;
>   	struct task_struct *task;
> @@ -1224,7 +1225,10 @@ static void update_tasks_cpumask(struct cpuset *cs)
>   		if (top_cs && (task->flags & PF_KTHREAD) &&
>   		    kthread_is_per_cpu(task))
>   			continue;
> -		set_cpus_allowed_ptr(task, cs->effective_cpus);
> +
> +		cpumask_and(new_cpus, cs->effective_cpus,
> +			    task_cpu_possible_mask(task));
> +		set_cpus_allowed_ptr(task, new_cpus);
>   	}
>   	css_task_iter_end(&it);
>   }
> @@ -1509,7 +1513,7 @@ static int update_parent_subparts_cpumask(struct cpuset *cs, int cmd,
>   	spin_unlock_irq(&callback_lock);
>   
>   	if (adding || deleting)
> -		update_tasks_cpumask(parent);
> +		update_tasks_cpumask(parent, tmp->new_cpus);
>   
>   	/*
>   	 * Set or clear CS_SCHED_LOAD_BALANCE when partcmd_update, if necessary.
> @@ -1661,7 +1665,7 @@ static void update_cpumasks_hier(struct cpuset *cs, struct tmpmasks *tmp,
>   		WARN_ON(!is_in_v2_mode() &&
>   			!cpumask_equal(cp->cpus_allowed, cp->effective_cpus));
>   
> -		update_tasks_cpumask(cp);
> +		update_tasks_cpumask(cp, tmp->new_cpus);
>   
>   		/*
>   		 * On legacy hierarchy, if the effective cpumask of any non-
> @@ -2309,7 +2313,7 @@ static int update_prstate(struct cpuset *cs, int new_prs)
>   		}
>   	}
>   
> -	update_tasks_cpumask(parent);
> +	update_tasks_cpumask(parent, tmpmask.new_cpus);
>   
>   	if (parent->child_ecpus_count)
>   		update_sibling_cpumasks(parent, cs, &tmpmask);
> @@ -3347,7 +3351,7 @@ hotplug_update_tasks_legacy(struct cpuset *cs,
>   	 * as the tasks will be migrated to an ancestor.
>   	 */
>   	if (cpus_updated && !cpumask_empty(cs->cpus_allowed))
> -		update_tasks_cpumask(cs);
> +		update_tasks_cpumask(cs, new_cpus);
>   	if (mems_updated && !nodes_empty(cs->mems_allowed))
>   		update_tasks_nodemask(cs);
>   
> @@ -3384,7 +3388,7 @@ hotplug_update_tasks(struct cpuset *cs,
>   	spin_unlock_irq(&callback_lock);
>   
>   	if (cpus_updated)
> -		update_tasks_cpumask(cs);
> +		update_tasks_cpumask(cs, new_cpus);
>   	if (mems_updated)
>   		update_tasks_nodemask(cs);
>   }

Acked-by: Waiman Long <longman@redhat.com>

This change is good for backporting to stable releases. For the latest 
kernel, I will prefer to centralize the masking in 
__set_cpus_allowed_ptr() where a scratch_mask is already being used for 
masking purpose. Let get this patch merged now, I will send a patch to 
move off the masking afterward.

Cheers,
Longman