linux-kernel - Re: Possible issue with commit 4961b6e11825?

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Message-ID: <20151206205603.GA9008@linux.vnet.ibm.com>
Date:	Sun, 6 Dec 2015 12:56:03 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	tglx@...utronix.de, peterz@...radead.org,
	preeti@...ux.vnet.ibm.com, viresh.kumar@...aro.org,
	mtosatti@...hat.com, fweisbec@...il.com
Cc:	linux-kernel@...r.kernel.org, sasha.levin@...cle.com
Subject: Re: Possible issue with commit 4961b6e11825?

On Sat, Dec 05, 2015 at 11:01:24AM -0800, Paul E. McKenney wrote:
> On Fri, Dec 04, 2015 at 03:20:22PM -0800, Paul E. McKenney wrote:
> > Hello!
> > 
> > Are there any known issues with commit 4961b6e11825 (sched: core: Use
> > hrtimer_start[_expires]())?
> > 
> > The reason that I ask is that I am about 90% sure that an rcutorture
> > failure bisects to that commit.  I will be running more tests on
> > 3497d206c4d9 (perf: core: Use hrtimer_start()), which is the predecessor
> > of 4961b6e11825, and which, unlike 4961b6e11825, passes a 12-hour
> > rcutorture test with scenario TREE03.  In contrast, 4961b6e11825 gets
> > 131 RCU CPU stall warnings, 132 reports of one of RCU's grace-period
> > kthreads being starved, and 525 reports of one of rcutorture's kthreads
> > being starved.  Most of the test runs hang on shutdown, which is no
> > surprise if an RCU CPU stall is happening at about that time.
> > 
> > But perhaps 3497d206c4d9 was just getting lucky, hence additional testing
> > over the weekend.
> 
> And it was getting lucky.  In a set of 24 two-hour runs (triple parallel)
> on an earlier commit (not 3497d206c4d9, no clue what I was thinking) got
> me two failed runs, for a total of 49 reports of one of RCU's grace-period
> kthreads being starved, no reports of rcutorture's kthreads being starved,
> and no hangs on shutdown.  So much lower failure rate, but still failures.
> 
> At this point, I am a bit disgusted with bisection, so my next test cycle
> (36 two-hour runs on a system capable of doing three concurrently) is on
> the most recent -rcu, but with CPU hotplug disabled.  If that shows failures,
> then I hammer 3497d206c4d9 hard.

And no failures on current -rcu with CPU hotplug disabled.  So this
seems to be specific to CPU hotplug.  So my next step is to fix some
remaining known CPU-hotplug issues in RCU.

And Thomas, when you get those CPU-hotplug patches ready, I have a testcase
for you!  ;-)

								Thanx, Paul

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