Re: Possible issue with commit 4961b6e11825?

From: Paul E. McKenney
Date: Sat Dec 05 2015 - 14:01:33 EST


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.

Anyway, if you have any ideas as to what might be happening, please don't
keep them a secret!

Thanx, Paul

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