Re: [PATCH v3] sched/core: Use empty mask to reset cpumasks in sched_setaffinity()

[Waiman Long <longman@redhat.com>]

On 8/3/23 22:32, Waiman Long wrote:
> Since commit 8f9ea86fdf99 ("sched: Always preserve the user requested
> cpumask"), user provided CPU affinity via sched_setaffinity(2) is
> perserved even if the task is being moved to a different cpuset. However,
> that affinity is also being inherited by any subsequently created child
> processes which may not want or be aware of that affinity.
>
> One way to solve this problem is to provide a way to back off from
> that user provided CPU affinity.  This patch implements such a scheme
> by using an empty cpumask to signal a reset of the cpumasks to the
> default as allowed by the current cpuset.
>
> Before this patch, passing in an empty cpumask to sched_setaffinity(2)
> will always return an -EINVAL error. With this patch, an alternative
> error of -ENODEV will be returned returned if sched_setaffinity(2)
> has been called before to set up user_cpus_ptr. In this case, the
> user_cpus_ptr that stores the user provided affinity will be cleared and
> the task's CPU affinity will be reset to that of the current cpuset. This
> alternative error code of -ENODEV signals that the no CPU is specified
> and, at the same time, a side effect of resetting cpu affinity to the
> cpuset default.
>
> If sched_setaffinity(2) has not been called previously, an EINVAL error
> will be returned with an empty cpumask just like before.  Tests or
> tools that rely on the behavior that an empty cpumask will return an
> error code will not be affected.
>
> We will have to update the sched_setaffinity(2) manpage to document
> this possible side effect of passing in an empty cpumask.
>
> Signed-off-by: Waiman Long <longman@redhat.com>

Ping.

Are there other concerns about this patch? I haven't seen any error 
report from kernel test robot so far.

Cheers,
Longman

> ---
>   kernel/sched/core.c | 42 +++++++++++++++++++++++++++++++++---------
>   1 file changed, 33 insertions(+), 9 deletions(-)
>
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index c52c2eba7c73..3ef7397f2a61 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -8317,7 +8317,12 @@ __sched_setaffinity(struct task_struct *p, struct affinity_context *ctx)
>   	}
>   
>   	cpuset_cpus_allowed(p, cpus_allowed);
> -	cpumask_and(new_mask, ctx->new_mask, cpus_allowed);
> +
> +	/* Default to cpus_allowed with NULL new_mask */
> +	if (ctx->new_mask)
> +		cpumask_and(new_mask, ctx->new_mask, cpus_allowed);
> +	else
> +		cpumask_copy(new_mask, cpus_allowed);
>   
>   	ctx->new_mask = new_mask;
>   	ctx->flags |= SCA_CHECK;
> @@ -8366,6 +8371,7 @@ __sched_setaffinity(struct task_struct *p, struct affinity_context *ctx)
>   
>   long sched_setaffinity(pid_t pid, const struct cpumask *in_mask)
>   {
> +	bool reset_cpumasks = cpumask_empty(in_mask);
>   	struct affinity_context ac;
>   	struct cpumask *user_mask;
>   	struct task_struct *p;
> @@ -8403,15 +8409,26 @@ long sched_setaffinity(pid_t pid, const struct cpumask *in_mask)
>   		goto out_put_task;
>   
>   	/*
> -	 * With non-SMP configs, user_cpus_ptr/user_mask isn't used and
> -	 * alloc_user_cpus_ptr() returns NULL.
> +	 * If an empty cpumask is passed in and user_cpus_ptr is set,
> +	 * clear user_cpus_ptr and reset the current cpu affinity to the
> +	 * default for the current cpuset. If user_cpus_ptr isn't set,
> +	 * -EINVAL will be returned as before.
>   	 */
> -	user_mask = alloc_user_cpus_ptr(NUMA_NO_NODE);
> -	if (user_mask) {
> -		cpumask_copy(user_mask, in_mask);
> -	} else if (IS_ENABLED(CONFIG_SMP)) {
> -		retval = -ENOMEM;
> -		goto out_put_task;
> +	if (reset_cpumasks && p->user_cpus_ptr) {
> +		in_mask = NULL;	/* To be updated in __sched_setaffinity */
> +		user_mask = NULL;
> +	} else {
> +		/*
> +		 * With non-SMP configs, user_cpus_ptr/user_mask isn't used
> +		 * and alloc_user_cpus_ptr() returns NULL.
> +		 */
> +		user_mask = alloc_user_cpus_ptr(NUMA_NO_NODE);
> +		if (user_mask) {
> +			cpumask_copy(user_mask, in_mask);
> +		} else if (IS_ENABLED(CONFIG_SMP)) {
> +			retval = -ENOMEM;
> +			goto out_put_task;
> +		}
>   	}
>   
>   	ac = (struct affinity_context){
> @@ -8423,6 +8440,13 @@ long sched_setaffinity(pid_t pid, const struct cpumask *in_mask)
>   	retval = __sched_setaffinity(p, &ac);
>   	kfree(ac.user_mask);
>   
> +	/*
> +	 * Force an error return (-ENODEV), if no error yet, for the empty
> +	 * cpumask case to avoid breaking existing tests.
> +	 */
> +	if (reset_cpumasks && !retval)
> +		retval = -ENODEV;
> +
>   out_put_task:
>   	put_task_struct(p);
>   	return retval;