Re: [PATCH v5 07/15] cpuset: Don't use the cpu_possible_mask as a last resort for cgroup v1

[Qais Yousef <qais.yousef@arm.com>]

On 12/08/20 13:28, Will Deacon wrote:
> If the scheduler cannot find an allowed CPU for a task,
> cpuset_cpus_allowed_fallback() will widen the affinity to cpu_possible_mask
> if cgroup v1 is in use.
> 
> In preparation for allowing architectures to provide their own fallback
> mask, just return early if we're not using cgroup v2 and allow
> select_fallback_rq() to figure out the mask by itself.
> 
> Cc: Li Zefan <lizefan@huawei.com>
> Cc: Tejun Heo <tj@kernel.org>
> Cc: Johannes Weiner <hannes@cmpxchg.org>
> Reviewed-by: Quentin Perret <qperret@google.com>
> Signed-off-by: Will Deacon <will@kernel.org>
> ---
>  kernel/cgroup/cpuset.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> index 57b5b5d0a5fd..e970737c3ed2 100644
> --- a/kernel/cgroup/cpuset.c
> +++ b/kernel/cgroup/cpuset.c
> @@ -3299,9 +3299,11 @@ void cpuset_cpus_allowed(struct task_struct *tsk, struct cpumask *pmask)
>  
>  void cpuset_cpus_allowed_fallback(struct task_struct *tsk)
>  {
> +	if (!is_in_v2_mode())
> +		return; /* select_fallback_rq will try harder */
> +
>  	rcu_read_lock();
> -	do_set_cpus_allowed(tsk, is_in_v2_mode() ?
> -		task_cs(tsk)->cpus_allowed : cpu_possible_mask);
> +	do_set_cpus_allowed(tsk, task_cs(tsk)->cpus_allowed);

Why is it safe to return that for cpuset v2? task_cs(tsk)->cpus_allowed is the
original user configured settings of the cpuset.cpus; which could have empty
intersection with task_cpu_possible_mask(), no?

do_set_cpus_allowed() will call set_cpus_allowed_common() which will end up
copying the mask as-is.

So unless I missed something there's a risk a 32bit task ends up having a 64bit
only cpu_mask when using cpuset v2.

Thanks

--
Qais Yousef

>  	rcu_read_unlock();
>  
>  	/*
> -- 
> 2.29.2.576.ga3fc446d84-goog
>