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    SubjectRe: [RFC] Make need_resched() return true when rcu_urgent_qs requested
    On Thu, Jul 12, 2018 at 09:17:04AM -0700, Paul E. McKenney wrote:
    > On Thu, Jul 12, 2018 at 05:53:51AM -0700, Paul E. McKenney wrote:
    > > On Thu, Jul 12, 2018 at 01:00:42PM +0100, David Woodhouse wrote:
    > > >
    > > >
    > > > On Wed, 2018-07-11 at 14:08 -0700, Paul E. McKenney wrote:
    > > > >
    > > > > > Also... why in $DEITY's name was the existing
    > > > > > rcu_virt_note_context_switch() not actually sufficient? If we had that
    > > > > > there, why did we need an additional explicit calls to rcu_all_qs() in
    > > > > > the KVM loop, or the more complex fixes to need_resched() which
    > > > > > ultimately had the same effect, to avoid ten-second latencies?
    > > > >
    > > > > My guess is that this was because control passed through the
    > > > > rcu_virt_note_context_switch() only once, and then subsequent
    > > > > scheduling-clock interrupts bypassed this code.
    > >
    > > Gah! My guess was instead that the code did a rcu_kvm_enter() going in,
    > > but somehow managed to miss the rcu_kvm_exit() going out. But that makes
    > > absolutely no sense -- had that happened, rcutorture would likely have
    > > screamed bloody murder, loudly and often. No mere near misses!
    > >
    > > And besides, thus far, -ENOREPRODUCE. :-/
    >
    > OK, one close call in 63 hours of rcutorture, this one on scenario TREE03
    > (yesterday hit TREE01 and TREE03). Time for probabilitistic long-runtime
    > bisection. Plus thought about how to get more information out of the near
    > misses. Fun! ;-)

    Most of the weekend was devoted to testing today's upcoming pull request,
    but I did get a bit more testing done on this.

    I was able to make this happen more often by tweaking rcutorture a
    bit, but I still do not yet have statistically significant results.
    Nevertheless, I have thus far only seen failures with David's patch or
    with both David's and my patch. And I actually got a full-up rcutorture
    failure (a too-short grace period) in addition to the aforementioned
    close calls.

    Over this coming week I expect to devote significant testing time to
    the commit just prior to David's in my stack. If I don't see failures
    on that commit, we will need to spent some quality time with the KVM
    folks on whether or not kvm_x86_ops->run() and friends have the option of
    failing to return, but instead causing control to pop up somewhere else.
    Or someone could tell me how I am being blind to some obvious bug in
    the two commits that allow RCU to treat KVM guest-OS execution as an
    extended quiescent state. ;-)

    Thanx, Paul

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