Re: [PATCH for-5.15 v2] io_uring: consider cgroup setting when binding sqpoll cpu

[Tejun Heo <tj@kernel.org>]

Hello,

On Fri, Aug 27, 2021 at 10:13:15PM +0800, Hao Xu wrote:
> +static int io_sq_bind_cpu(int cpu)
> +{
> +	if (!test_cpu_in_current_cpuset(cpu))
> +		pr_warn("sqthread %d: bound cpu not allowed\n", current->pid);
> +	else
> +		set_cpus_allowed_ptr(current, cpumask_of(cpu));
> +
> +	return 0;
> +}
...
> @@ -8208,8 +8217,10 @@ static int io_sq_offload_create(struct io_ring_ctx *ctx,
>  			int cpu = p->sq_thread_cpu;
>  
>  			ret = -EINVAL;
> -			if (cpu >= nr_cpu_ids || !cpu_online(cpu))
> +			if (cpu >= nr_cpu_ids || !cpu_online(cpu) ||
> +			    !test_cpu_in_current_cpuset(cpu))
>  				goto err_sqpoll;

Given that sq_thread is user-like thread and belongs to the right cgroup,
I'm not quite sure what the above achieves - the affinities can break
anytime, so one-time check doesn't really solve the problem. All it seems to
add is warning messages. What's the expected behavior when an io thread
can't run on the target cpu anymore?

Thanks.

-- 
tejun