linux-kernel - Re: No freezing of kernel threads (was: Re: [GIT PULL] libata fixes for v3.13-rc5)

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Date:	Thu, 26 Dec 2013 21:21:42 -0500
From:	Tejun Heo <tj@...nel.org>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Kernel development list <linux-kernel@...r.kernel.org>,
	linux-ide@...r.kernel.org,
	Linux-pm mailing list <linux-pm@...r.kernel.org>
Subject: Re: No freezing of kernel threads (was: Re: [GIT PULL] libata fixes
 for v3.13-rc5)

Hello,

On Thu, Dec 26, 2013 at 09:14:49PM -0500, Alan Stern wrote:
> I can't disagree with this.  But the design may well be perfectly
> adequate for some use cases.  Given a workqueue or kthread which should
> not operate during system sleep, we have to:
> 
> 	Tell the wq/thread to stop running because a sleep is about
> 	to start, and
> 
> 	Provide a function the wq/thread can call to put itself on 
> 	hold for the duration of the sleep.
> 
> The freezer does both these things pretty efficiently.  Problems may

I don't even like the interface itself.  It's too implicit and spread
all over the place - we basically had to spread it all over the wait
interfaces, and the implementation is far more involved than called
for.  I don't know how you're defining "efficiently" but that isn't a
word I'd use to describe the freezer.

> So you're suggesting changing the kthread to a workqueue thread, but
> keeping the existing list of scheduled events instead of relying on the
> workqueue's own queue of work items?  What's the advantage?  Making
> such a change wouldn't simplify anything.

Well, that'd be an easy first step which does away with the dedicated
kthread, likely reduces cache footprint and allows use of wq
synchronization / freezing constructs which are easier to deal with.
More involved conversion would of course be doable but even simple
conversion seems like a win.

Thanks.

-- 
tejun
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