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

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Message-ID: <20151206051841.GR28602@linux.vnet.ibm.com>
Date:	Sat, 5 Dec 2015 21:18:41 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Viresh Kumar <viresh.kumar@...aro.org>
Cc:	tglx@...utronix.de, peterz@...radead.org,
	preeti@...ux.vnet.ibm.com, mtosatti@...hat.com, fweisbec@...il.com,
	linux-kernel@...r.kernel.org, sasha.levin@...cle.com
Subject: Re: Possible issue with commit 4961b6e11825?

On Sun, Dec 06, 2015 at 08:06:47AM +0530, Viresh Kumar wrote:
> On 05-12-15, 11:01, Paul E. McKenney wrote:
> > 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.
> > 
> > Anyway, if you have any ideas as to what might be happening, please don't
> > keep them a secret!
> 
> I can be the least helpful here (based on knowledge), but I am not
> able to find a reason for this diff in 3497d206c4d9:
> 
> -       if (!hrtimer_callback_running(hr))
> -               __hrtimer_start_range_ns(hr, cpuctx->hrtimer_interval,
> -                                        0, HRTIMER_MODE_REL_PINNED, 0);
> +       hrtimer_start(hr, cpuctx->hrtimer_interval, HRTIMER_MODE_REL_PINNED);
> 
> The commit talks *only* about s/__hrtimer_start_range_ns/hrtimer_start
> but not at all on why !hrtimer_callback_running(hr) was removed.
> Perhaps there was a reason :)

It is quite possible that this commit was an innocent bystander.  I will
know more in about 16 hours after the current round of tests complete.
These hammer current -rcu, but with CPU hotplug Kconfig'ed out.

							Thanx, Paul

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