Re: [PATCH 7/7] memcg watermark reclaim workqueue.

[KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>]

On Tue, 26 Apr 2011 16:19:41 -0700
Ying Han <yinghan@google.com> wrote:

> On Mon, Apr 25, 2011 at 2:42 AM, KAMEZAWA Hiroyuki <
> kamezawa.hiroyu@jp.fujitsu.com> wrote:

> > @@ -3661,6 +3683,67 @@ unsigned long mem_cgroup_soft_limit_recl
> >        return nr_reclaimed;
> >  }
> >
> > +struct workqueue_struct *memcg_bgreclaimq;
> > +
> > +static int memcg_bgreclaim_init(void)
> > +{
> > +       /*
> > +        * use UNBOUND workqueue because we traverse nodes (no locality)
> > and
> > +        * the work is cpu-intensive.
> > +        */
> > +       memcg_bgreclaimq = alloc_workqueue("memcg",
> > +                       WQ_MEM_RECLAIM | WQ_UNBOUND | WQ_FREEZABLE, 0);
> > +       return 0;
> > +}
> >
> 
> I read about the documentation of workqueue. So the WQ_UNBOUND support the
> max 512 execution contexts per CPU. Does the execution context means thread?
> 
> I think I understand the motivation of that flag, so we can have more
> concurrency of bg reclaim workitems. But one question is on the workqueue
> scheduling mechanism. If we can queue the item anywhere as long as they are
> inserted in the queue, do we have mechanism to support the load balancing
> like the system scheduler? The scenario I am thinking is that one CPU has
> 512 work items and the other one has 1.
> 
IIUC, UNBOUND workqueue doesn't have cpumask and it can be scheduled anywhere.
So, scheduler's load balancing works well.

Because unbound_gcwq_nr_running == 0 always (If I believe comment on source),
 __need_more_worker() always returns true and 
need_to_create_worker() returns true if no idle thread.

Then, I think new kthread is created always if there is a work.

I wonder I shoud use WQ_CPU_INTENSIVE and spread jobs to each cpu per memcg. But
I don't see problem with UNBOUND wq, yet.


> I don't think this is directly related issue for this patch, and I just hope
> the workqueue mechanism already support something like that for load
> balancing.
> 
If not, we can add it.

Thanks,
-Kame

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