linux-kernel - Re: [PATCH 2/6] workqueue: add support for lazy workqueues

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Date:	Mon, 24 Aug 2009 10:06:26 +0200
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	linux-kernel@...r.kernel.org, jeff@...zik.org,
	benh@...nel.crashing.org, htejun@...il.com, bzolnier@...il.com,
	alan@...rguk.ukuu.org.uk
Subject: Re: [PATCH 2/6] workqueue: add support for lazy workqueues

On Thu, Aug 20 2009, Andrew Morton wrote:
> On Thu, 20 Aug 2009 12:17:39 +0200
> Jens Axboe <jens.axboe@...cle.com> wrote:
> 
> > Lazy workqueues are like normal workqueues, except they don't
> > start a thread per CPU by default. Instead threads are started
> > when they are needed, and exit when they have been idle for
> > some time.
> > 
> >
> > ...
> >
> > @@ -280,7 +309,34 @@ static void run_workqueue(struct cpu_workqueue_struct *cwq)
> >  		trace_workqueue_execution(cwq->thread, work);
> >  		cwq->current_work = work;
> >  		list_del_init(cwq->worklist.next);
> > +		cpu = smp_processor_id();
> >  		spin_unlock_irq(&cwq->lock);
> > +		did_work = 1;
> > +
> > +		/*
> > +		 * If work->cpu isn't us, then we need to create the target
> > +		 * workqueue thread (if someone didn't already do that) and
> > +		 * move the work over there.
> > +		 */
> > +		if ((cwq->wq->flags & WQ_F_LAZY) && work->cpu != cpu) {
> > +			struct cpu_workqueue_struct *__cwq;
> > +			struct task_struct *p;
> > +			int err;
> > +
> > +			__cwq = wq_per_cpu(cwq->wq, work->cpu);
> > +			p = __cwq->thread;
> > +			if (!p)
> > +				err = create_workqueue_thread(__cwq, work->cpu);
> > +			p = __cwq->thread;
> > +			if (p) {
> > +				if (work->cpu >= 0)
> 
> It's an unsigned int.  This test is always true.
> 
> > +					kthread_bind(p, work->cpu);
> 
> I wonder what happens if work->cpu isn't online any more.

That's a good question. The workqueue "documentation" states that it is
the callers responsibility to ensure that the CPU stays online, but I
think that requirement is pretty much ignored. Probably since it'd be
costly to do.

So that bits needs looking into.

-- 
Jens Axboe

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