Re: [PATCH 2/3] workqueue: Add an API to create a singlethread workqueue attached to the current task's cgroup

On Thu, 2010-05-27 at 14:44 +0200, Oleg Nesterov wrote:

On 05/27, Michael S. Tsirkin wrote:

quoted

On Tue, May 18, 2010 at 05:04:51PM -0700, Sridhar Samudrala wrote:

quoted

Add a new kernel API to create a singlethread workqueue and attach it's
task to current task's cgroup and cpumask.

Signed-off-by: Sridhar Samudrala <redacted>

Could someone familiar with workqueue code please comment on whether
this patch is suitable for 2.6.35?

It is needed to fix the case where vhost user might cause a kernel
thread to consume more CPU than allowed by the cgroup.
Should I merge it through the vhost tree?
Ack for this?

I don't understand the reasons for this patch, but this doesn't matter.

I don't really see any need to change workqueue.c,

quoted

quoted

+static struct task_struct *get_singlethread_wq_task(struct workqueue_struct *wq)
+{
+	return (per_cpu_ptr(wq->cpu_wq, singlethread_cpu))->thread;
+}

(Not sure this trivial static helper with the single caller makes sense, but
 see below)

quoted

quoted

+/* Create a singlethread workqueue and attach it's task to the current task's
+ * cgroup and set it's cpumask to the current task's cpumask.
+ */
+struct workqueue_struct *create_singlethread_workqueue_in_current_cg(char *name)
+{
+	struct workqueue_struct *wq;
+	struct task_struct *task;
+	cpumask_var_t mask;
+
+	wq = create_singlethread_workqueue(name);
+	if (!wq)
+		goto out;
+
+	if (!alloc_cpumask_var(&mask, GFP_KERNEL))
+		goto err;
+			
+	if (sched_getaffinity(current->pid, mask))
+		goto err;
+
+	task = get_singlethread_wq_task(wq);
+	if (sched_setaffinity(task->pid, mask))
+		goto err;
+
+	if (cgroup_attach_task_current_cg(task))
+		goto err;
+out:	
+	return wq;
+err:
+	destroy_workqueue(wq);
+	wq = NULL;
+	goto out;
+}

Instead, cgroup.c (or whoever needs this) can do

	struct move_struct {
		struct work_struct work;
		int ret;
	};

	static void move_func(struct work_struct *work)
	{
		struct move_struct *move = container_of(...);

		if (cgroup_attach_task_current_cg(current))

We are trying to attach the task associated with workqueue to the
current task's cgroup. So what we need is 
   cgroup_attach_task_current_cg(wq->task);

However there is no interface currently that exports the task associated
with a workqueue. It is hidden in cpu_workqueue_struct and is only 
accessible within workqueue.c.

			ret = -EANY;
	}

	static struct workqueue_struct *create_singlethread_workqueue_in_current_cg(char *name)
	{
		struct workqueue_struct *wq;
		struct move_struct move = {
			.work = __WORK_INITIALIZER(move_func);
		};

		wq = create_singlethread_workqueue(name);
		if (!wq)
			return NULL;

		queue_work(&move.work);
		flush_work(&move.work);

		if (move.ret) {
			destroy_workqueue(wq);
			wq = NULL;
		}

		return wq;
	}

Or. Just export wq_per_cpu() from workqueue.c (probably with a better name) and
use it like the patch does.

This requires that struct cpu_workqueue_struct and struct
workqueue_struct are made externally visible by moving them to
kernel/workqueue.h.

Instead what about adding the simple helper get_singlethread_wq_task()
in workqueue.c and exporting it.
I can add create_singlethread_workqueue_in_current_cg() to cgroup.c
using this helper routine.

Thanks
Sridhar