linux-kernel - Re: [patch] workqueue: schedule WORK_CPU_UNBOUND work on wq_unbound

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Date:	Fri, 24 Jul 2015 05:38:59 +0200
From:	Mike Galbraith <umgwanakikbuti@...il.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	Lai Jiangshan <jiangshanlai@...il.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Daniel Bristot de Oliveira <bristot@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Rik van Riel <riel@...hat.com>,
	"Luis Claudio R. Goncalves" <lclaudio@...g.org>
Subject: Re: [patch] workqueue: schedule WORK_CPU_UNBOUND work on
 wq_unbound_cpumask CPUs

On Wed, 2015-07-22 at 11:25 -0400, Tejun Heo wrote:
> Hello,
> 
> On Wed, Jul 22, 2015 at 05:19:00PM +0200, Mike Galbraith wrote:
> > Ew, looking at the numbers, they may prefer to either a) pretend to not
> > notice, or b) scurry off to HPC'R'US store if a) won't fly ;-)
> 
> Yeah, there are a lot of them.  The sad part is that only very few of
> them would actually need local binding for correctness. :(

I'm going to end this thread with an under my breath mutter.  Before I
do that, note that I went and beat up three different boxen over days
before turning trivial little patchlet loose, ie I was quite paranoid
despite the below, "No way Jose" was not entirely unexpected.

Mutter: WORK_CPU_UNBOUND is about as far from a local execution
guarantee as it gets, and not only is it prominently displayed for all
to see...

/**
 * queue_work - queue work on a workqueue
 * @wq: workqueue to use
 * @work: work to queue
 *
 * Returns %false if @work was already on a queue, %true otherwise.
 *
 * We queue the work to the CPU on which it was submitted, but if the CPU dies
 * it can be processed by another CPU.
 */
static inline bool queue_work(struct workqueue_struct *wq,
                              struct work_struct *work)
{
        return queue_work_on(WORK_CPU_UNBOUND, wq, work);
}

...there's nothing ambiguous about that.  While it states that we queue
locally, it also clearly states that local execution is NOT guaranteed,
making assumption thereof a BUG, which in turn makes it seem a bit odd
to tie fixing up nohz_full a little to auditing and fixing every booboo
ever made (unlikely to ever happen).  It would appear that the only way
anyone could ever have sanely assume local execution, WORK_CPU_UNBOUND
aside, is to have called get_online_cpus() before queueing any work.

Mutter expressed, it's not a big hairy deal, anyone using an SMP kernel
with an expectation of zero perturbation is doomed to be disappointed
anyway, patchlet just reduced noise a bit.

	-Mike (mutter mutter mutter;)

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