linux-kernel - Re: [GIT PULL] workqueue fixes for v4.3-rc5

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Date:	Wed, 14 Oct 2015 16:24:48 -0400
From:	Tejun Heo <tj@...nel.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Christoph Lameter <cl@...ux.com>,
	Michal Hocko <mhocko@...e.cz>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Lai Jiangshan <jiangshanlai@...il.com>,
	Shaohua Li <shli@...com>, linux-mm <linux-mm@...ck.org>
Subject: Re: [GIT PULL] workqueue fixes for v4.3-rc5

Hello,

On Wed, Oct 14, 2015 at 01:10:33PM -0700, Linus Torvalds wrote:
> At the same time, some of the same issues that are pushing people to
> move timers around (put idle cores to deeper sleeps etc) would also
> argue for moving delayed work around to other cpus if possible.
> 
> So I agree that there is a push to make timer cpu targets more dynamic
> in a way we historically didn't really have. At the same time, I think
> the same forces that want to move timers around would actually likely
> want to move delayed work around too...

I fully agree.  We gotta get this in order sooner or later.  I'll try
to come up with a transition plan.

> > * This makes queue_delayed_work() behave differently from queue_work()
> >   and when I checked years ago the local queueing guarantee was
> >   definitely being depended upon by some users.
> 
> Yes. But the delayed work really is different. By definition, we know
> that the current cpu is busy and active _right_now_, and so keeping
> work on that cpu isn't obviously wrong.
> 
> But it's *not* obviously right to schedule something on that
> particular cpu a few seconds from now, when it might be happily asleep
> and there might be better cpus to bother..

But in terms of API consistency, it sucks to have queue_work()
guarantee local queueing but not queue_delayed_work().  The ideal
situation would be updating both so that neither guarantees.  If that
turns out to be too painful, maybe we can rename queue_delayed_work()
so that it signifies its difference from queue_work().  Let's see.

> > I do want to get rid of the local queueing guarnatee for all work
> > items.  That said, I don't think this is the right way to do it.
> 
> Hmm. I guess that for being past rc5, taking your patch is the safe
> thing. I really don't like it very much, though.

Heh, yeah, I pondered about calling it a happy accident and just
sticking with the new behavior.

Thanks.

-- 
tejun
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