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@amazon.de Reported-by: David Woodhouse dwmw2@infradead.org Suggested-by: Peter Zijlstra peterz@infradead.org Signed-off-by: Paul E. McKenney paulmck@linux.vnet.ibm.com [ paulmck: Backported to make patch apply cleanly on older versions. ] Tested-by: Marius Hillenbrand mhillenb@amazon.de Cc: stable@vger.kernel.org # 4.12.x - 4.19.x
Now queued up, thanks.
greg k-h