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    SubjectRe: 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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