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

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Date:	Tue, 14 Aug 2012 02:43:28 +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()

>> 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.

I got your point.
Thanks for kind illustration.

Following is a alternative implementation for this.
I thinks this is too rare case, so it doesn't help in any real workload.
But how do you thinks?

@@ -1156,7 +1156,9 @@ int queue_delayed_work_on(int cpu, struct
workqueue_struct *wq,
                if (!(wq->flags & WQ_UNBOUND)) {
                        struct global_cwq *gcwq = get_work_gcwq(work);

-                       if (gcwq && gcwq->cpu != WORK_CPU_UNBOUND)
+                       if (!gcwq)
+                               lcpu = cpu;
+                       else if (gcwq->cpu != WORK_CPU_UNBOUND)
                                lcpu = gcwq->cpu;
                        else
                                lcpu = raw_smp_processor_id();
--
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