On Sun, Nov 04, 2018 at 06:34:34PM -0800, Paul E. McKenney wrote: > commit 92aa39e9dc77 upstream. > > The per-CPU rcu_dynticks.rcu_urgent_qs variable communicates an urgent > need for an RCU quiescent state from the force-quiescent-state processing > within the grace-period kthread to context switches and to cond_resched(). > Unfortunately, such urgent needs are not communicated to need_resched(), > which is sometimes used to decide when to invoke cond_resched(), for > but one example, within the KVM vcpu_run() function. As of v4.15, this > can result in synchronize_sched() being delayed by up to ten seconds, > which can be problematic, to say nothing of annoying. > > This commit therefore checks rcu_dynticks.rcu_urgent_qs from within > rcu_check_callbacks(), which is invoked from the scheduling-clock > interrupt handler. If the current task is not an idle task and is > not executing in usermode, a context switch is forced, and either way, > the rcu_dynticks.rcu_urgent_qs variable is set to false. If the current > task is an idle task, then RCU's dyntick-idle code will detect the > quiescent state, so no further action is required. Similarly, if the > task is executing in usermode, other code in rcu_check_callbacks() and > its called functions will report the corresponding quiescent state. > > Reported-by: Marius Hillenbrand <mhillenb@xxxxxxxxx> > Reported-by: David Woodhouse <dwmw2@xxxxxxxxxxxxx> > Suggested-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx> > Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> > [ paulmck: Backported to make patch apply cleanly on older versions. ] > Tested-by: Marius Hillenbrand <mhillenb@xxxxxxxxx> > Cc: <stable@xxxxxxxxxxxxxxx> # 4.12.x - 4.19.x > Now queued up, thanks. greg k-h
