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

[Ying Han <yinghan@google.com>]

On Tue, Apr 26, 2011 at 5:31 PM, KAMEZAWA Hiroyuki
<kamezawa.hiroyu@jp.fujitsu.com> wrote:
> 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.

Ah, ok. Then this works better than I thought, so we can use the
scheduler to put threads onto the
CPUs.

>
> 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 think the UNBOUND is good to start with.

>
>
>> 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.

So, you might already answered my question. The load balancing is done
by the system
scheduler since we fork new thread for the queued items.

--Ying
>
> Thanks,
> -Kame
>
>

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