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    SubjectRe: [RFC] Make need_resched() return true when rcu_urgent_qs requested
    On Fri, Jul 06, 2018 at 10:11:50AM -0700, Paul E. McKenney wrote:
    > On Fri, Jul 06, 2018 at 06:29:05PM +0200, Peter Zijlstra wrote:
    > > On Fri, Jul 06, 2018 at 03:53:30PM +0100, David Woodhouse wrote:
    > > > diff --git a/include/linux/sched.h b/include/linux/sched.h
    > > > index e4d4e60..89f5814 100644
    > > > --- a/include/linux/sched.h
    > > > +++ b/include/linux/sched.h
    > > > @@ -1616,7 +1616,8 @@ static inline int spin_needbreak(spinlock_t *lock)
    > > >
    > > > static __always_inline bool need_resched(void)
    > > > {
    > > > - return unlikely(tif_need_resched());
    > > > + return unlikely(tif_need_resched()) ||
    > > > + rcu_urgent_qs_requested();
    > > > }
    > >
    > > Instead of making need_resched() touch two cachelines, I think I would
    > > prefer adding resched_cpu() to rcu_request_urgent_qs_task().
    >
    > I used to do something like this, but decided that whacking each holdout
    > CPU over the head ten times a second was a bit much.

    This is only called from the !list_empty(rcu_tasks_holdout) loop in
    rcu_tasks_kthread afaict, and that has a
    schedule_timeout_interruptible(HZ) in it, which I read as once a second.

    Which seems like an entirely reasonable amount of time to kick a task.
    Not scheduling for a second is like an eternity.

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