linux-kernel - Re: workqueue code needing preemption disabled

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Message-ID: <1363624883.25967.184.camel@gandalf.local.home>
Date:	Mon, 18 Mar 2013 12:41:23 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Tejun Heo <tj@...nel.org>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	RT <linux-rt-users@...r.kernel.org>,
	Clark Williams <clark@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: workqueue code needing preemption disabled

On Mon, 2013-03-18 at 09:27 -0700, Tejun Heo wrote:

> Does that mean that a task holding gcwq->lock may be preempted?  If
> so, that sure could lead to weird problems.  Maybe gcwq->lock should
> be marked non-preemptible somehow?

If the gcwq->lock is never held for a long time (really, more than a
microsecond on today's processors is considered a long time), and it
does not nest any other spin_locks (raw locks are OK, like the rq lock).
Then we could mark the gcwq->lock as raw as well.

This would require the struct global_cwq lock to have:

	raw_spinlock_t		lock;

and then you would need to do:

s/spin_/raw_spin/ for all gcwq->lock usages.

But, I'm worried about the loops that are done while holding this lock.
Just looking at is_chained_work() that does for_each_busy_worker(), how
big can that list be? If it's bound by # of CPUs then that may be fine,
but if it can be as big as the # of workers assigned, with no real
limit, then its not fine, because that creates an unbound (non
deterministic) latency.

-- Steve

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