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

[Sridhar Samudrala <sri@us.ibm.com>]

On Thu, 2010-05-27 at 14:44 +0200, Oleg Nesterov wrote:
> On 05/27, Michael S. Tsirkin wrote:
> >
> > On Tue, May 18, 2010 at 05:04:51PM -0700, Sridhar Samudrala wrote:
> > > 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 <sri@us.ibm.com>
> >
> > 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,
> 
> > > +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)
> 
> > > +/* 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