Re: [PATCH 2/2] io_uring: consider cgroup setting when binding sqpoll cpu

在 2021/9/2 上午12:41, Tejun Heo 写道:
Hi Tejun,

Hello,

On Wed, Sep 01, 2021 at 08:43:22PM +0800, Hao Xu wrote:

quoted

@@ -7112,11 +7113,9 @@ static int io_sq_thread(void *data)
  
  	snprintf(buf, sizeof(buf), "iou-sqp-%d", sqd->task_pid);
  	set_task_comm(current, buf);
+	if (sqd->sq_cpu != -1 && test_cpu_in_current_cpuset(sqd->sq_cpu))
  		set_cpus_allowed_ptr(current, cpumask_of(sqd->sq_cpu));
+

Would it make sense to just test whether set_cpus_allowed_ptr() succeeded
afterwards?

Do you mean: if (sqd->sq_cpu != -1 && !set_cpus_allowed_ptr(current, cpumask_of(sqd->sq_cpu)))

I'm not familiar with set_cpus_allowed_ptr(), you mean it contains the
similar logic of test_cpu_in_current_cpuset?

quoted

@@ -8310,8 +8309,10 @@ 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;
+

Failing operations on transient conditions like this may be confusing. Let's
ignore cpuset for now. CPU hotplug is sometimes driven automatically for
power saving purposes, so failing operation based on whether a cpu is online
means that the success or failure of the operation can seem arbitrary. If
the operation takes place while the cpu happens to be online, it succeeds
and the thread gets unbound and rebound automatically as the cpu goes

This is a bit beyond of my knowledge, so you mean if the cpu back
online, the task will automatically schedule to this cpu? if it's true,
I think the code logic here is fine.

offline and online. If the operation takes place while the cpu happens to be
offline, the operation fails.

It's ok that it fails, we leave the option of retry to users themselves.

I don't know what the intended behavior here should be and we haven't been
pretty bad at defining reasonable behavior around cpu hotplug, so it'd
probably be worthwhile to consider what the behavior should be.

Thanks.

Thanks,
Hao