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

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Message-ID: <20120813170303.GF9180@google.com>
Date:	Mon, 13 Aug 2012 10:03:03 -0700
From:	Tejun Heo <tj@...nel.org>
To:	JoonSoo Kim <js1304@...il.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/5] workqueue: change value of lcpu in
 queue_delayed_work_on()

Hello,

On Tue, Aug 14, 2012 at 01:54:22AM +0900, JoonSoo Kim wrote:
> 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?

Nope, that sound correct to me.

> And, do u mean  @cpu is WORK_CPU_UNBOUND?

@cpu could be WORK_CPU_UNBOUND at that point.  The timer will be added
to local CPU but @work->data would be pointing to WORK_CPU_UNBOUND,
again triggering the condition.  Given that @cpu being
WORK_CPU_UNBOUND is far more common than an actual CPU number, the
patch would actually increase spurious nrt lookups.  The right thing
to do is probably setting cpu to raw_smp_processor_id() beforehand.

Thanks.

-- 
tejun
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