linux-kernel - Re: [PATCH 2/5] workqueue: change value of lcpu in queue_delayed_work

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Date:	Tue, 14 Aug 2012 01:54:22 +0900
From:	JoonSoo Kim <js1304@...il.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/5] workqueue: change value of lcpu in queue_delayed_work_on()

2012/8/14 Tejun Heo <tj@...nel.org>:
> Hello,
>
> On Tue, Aug 14, 2012 at 01:17:49AM +0900, Joonsoo Kim wrote:
>> We assign cpu id into work struct in queue_delayed_work_on().
>> In current implementation, when work is come in first time,
>> current running cpu id is assigned.
>> If we do queue_delayed_work_on() with CPU A on CPU B,
>> __queue_work() invoked in delayed_work_timer_fn() go into sub-optimal path
>> in case of WQ_NON_REENTRANT.
>> Change it to cpu argument is prevent to go into sub-optimal path.
>
> Which part is suboptimal?  Also, what if @cpu is WQ_UNBOUND?

Hi.
I think a following scenario.

wq = WQ_NON_REENTRANT.
queue_delayed_work_on(CPU B) is invoked in CPU A, so lcpu = CPU A, cpu = CPU B.

In this case, we call add_time_on(CPU B), then delayed_work_timer_fn()
is invoked on CPU B.
delayed_work_timer_fn() calls __queue_work(), then following
comparisons return true!

                gcwq = get_gcwq(cpu);
                if (wq->flags & WQ_NON_REENTRANT &&
                    (last_gcwq = get_work_gcwq(work)) && last_gcwq != gcwq) {

I thinks that if we assign cpu to lcpu, above comparisons return
false, so save some overheads.
Is there any missing part?

And, do u mean  @cpu is WORK_CPU_UNBOUND?

Thanks.
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