linux-kernel - Re: workqueue code needing preemption disabled

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Message-ID: <20130318164610.GB21516@mtj.dyndns.org>
Date:	Mon, 18 Mar 2013 09:46:10 -0700
From:	Tejun Heo <tj@...nel.org>
To:	Steven Rostedt <rostedt@...dmis.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, Mar 18, 2013 at 12:41:23PM -0400, Steven Rostedt wrote:
> 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.

In most paths, gcwq->lock shouldn't be held for too long but yes there
are cold paths which just do things without thinking about latency
issues.  is_chained_work() can definitely take pretty long time (note
that it got reimplemented in the current devel branch and the loop is
gone).

-- 
tejun
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