linux-kernel - Re: [PATCH 5/5] workqueue: Allow modifying low level unbound workqueue cpumask

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Date:	Tue, 20 May 2014 22:08:30 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Christoph Lameter <cl@...ux.com>,
	Kevin Hilman <khilman@...aro.org>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	Mike Galbraith <bitbucket@...ine.de>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Viresh Kumar <viresh.kumar@...aro.org>
Subject: Re: [PATCH 5/5] workqueue: Allow modifying low level unbound
 workqueue cpumask

On Tue, May 20, 2014 at 03:56:56PM -0400, Tejun Heo wrote:
> > > Hmmm... but there's nothing which makes rolling back more likely to
> > > succeed compared to the original applications.  It's gonna allocate
> > > more pwqs.  Triggering WARN_ON_ONCE() seems weird.
> > 
> > Yeah but that's the least we can do. If we fail to even recover the old cpumask,
> > the user should know about the half state fail.
> 
> I'm failing to see how it'd be better than just going through applying
> the new mask if we're likely to end up with half-updated states
> anyway.  What's the point of another layer of best effort logic which
> is more likely to fail?

If the error is -ENOMEM then yeah, but any other error wants rollback.

> > But it's going to imply fun with double linked list of struct pwq_allocation_object
> > and stuff. Or maybe an array. This reminds be a bit generate_sched_domains(). It's
> > not going to be _that_ simple nor pretty :)
> 
> Is it tho?  Don't we just need to keep a separate staging copy of
> prepared pwq_tbl?  The commit stage can be pwq_tbl installation.
> Looks like it shouldn't be too much of problem.  Am I missing
> something?

Sure, that still need an iteration array/list of pre-allocated objects.
Expect at least one more hundred lines.

> 
> > > 2. Proper error handling is hard.  Just do pr_warn() on each failure
> > >    and continue to try to apply and always return 0.
> > > 
> > > If #1 isn't too complicated (would it be?), it'd be the better option;
> > > otherwise, well, #2 should work most of the time, eh?
> > 
> > Yeah I think #2 should be way enough 99% of the time :)
> 
> Yeah, if #1 gets too hairy, #2 can be a reluctant option but if #1 is
> doable without too much complication, I'd much prefer proper error
> handling.

I can try yeah.
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