linux-kernel - Re: rcu_sched stalls in idle task introduced in pre-4.8?

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Date:	Wed, 3 Aug 2016 09:16:31 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Rich Felker <dalias@...c.org>
Cc:	linux-kernel@...r.kernel.org, john.stultz@...aro.org,
	tglx@...utronix.de
Subject: Re: rcu_sched stalls in idle task introduced in pre-4.8?

On Tue, Aug 02, 2016 at 01:45:04PM -0700, Paul E. McKenney wrote:
> On Tue, Aug 02, 2016 at 04:32:17PM -0400, Rich Felker wrote:
> > On Tue, Aug 02, 2016 at 12:48:02PM -0700, Paul E. McKenney wrote:

[ . . . ]

> > > Does the problem reproduces easily?
> > 
> > Yes, it happens right after boot and repeats every 30-90 seconds or
> > so.
> 
> Well, that at least makes it easier to test any patches!
> 
> > > A bisection might be very helpful.
> > 
> > Bisection would require some manual work to setup because the whole
> > reason I was rebasing on Linus's tree was to adapt the drivers to
> > upstream infrastructure changes (the new cpuhp stuff replacing
> > notifier for cpu starting). The unfortunate way it was done, each
> > driver adds an enum to linux/cpuhotplug.h so all the patches have
> > gratuitous conflicts. In addition, for older revisions in Linus's
> > tree, there's at least one show-stopping (hang during boot) bug that
> > needs a cherry-pick to fix. There may be other small issues too. I
> > don't think they're at all insurmountible but it requires an annoying
> > amount of scripting.
> 
> I had to ask!  Might eventually be necessary, but let's see what we
> can learn from what you currently have.

And at first glance, my overnight run looks uglier than I would expect.
I am now running tests at v4.7, and will run other tests to see if
there really is a statistically significant degradation.  If there is,
then I might be able to bisect, though with nine-hour runs this could
take quite some time.

							Thanx, Paul