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rcu: Make need_resched() respond to urgent RCU-QS needs
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>
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@ -2471,6 +2471,15 @@ void rcu_check_callbacks(int user)
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{
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trace_rcu_utilization(TPS("Start scheduler-tick"));
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raw_cpu_inc(rcu_data.ticks_this_gp);
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/* The load-acquire pairs with the store-release setting to true. */
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if (smp_load_acquire(this_cpu_ptr(&rcu_dynticks.rcu_urgent_qs))) {
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/* Idle and userspace execution already are quiescent states. */
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if (!is_idle_task(current) && !user) {
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set_tsk_need_resched(current);
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set_preempt_need_resched();
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}
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__this_cpu_write(rcu_dynticks.rcu_urgent_qs, false);
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}
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rcu_flavor_check_callbacks(user);
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if (rcu_pending())
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invoke_rcu_core();
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