Make need_resched() respond to urgent RCU-QS needs
Subsystems (2)
PATCH 1/1 Make need_resched() respond to urgent RCU-QS needs
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.
kernel/rcu/tree.c +9 -0
@@ -2662,6 +2662,15 @@ void rcu_check_callbacks(int user)rcu_bh_qs (); }rcu_preempt_check_callbacks (); +/* The load-acquire pairs with the store-release setting to true. */ +if (smp_load_acquire (this_cpu_ptr (&rcu_dynticks.rcu_urgent_qs))) { +/* Idle and userspace execution already are quiescent states. */ +if (!rcu_is_cpu_rrupt_from_idle () && !user) { +set_tsk_need_resched (current); +set_preempt_need_resched (); + } +__this_cpu_write (rcu_dynticks.rcu_urgent_qs, false); + }if (rcu_pending ())invoke_rcu_core ();
Quoted 21 lines · Paul E. McKenney
commit
92aa39e9dc77upstream.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 toneed_resched(), which is sometimes used to decide when to invokecond_resched(), for but one example, within the KVMvcpu_run()function. As of v4.15, this can result insynchronize_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 inrcu_check_callbacks()and its called functions will report the corresponding quiescent state.
Quoted 8 lines
Reported-by Marius Hillenbrand <[email protected]>Reported-by David Woodhouse <[email protected]>Suggested-by Peter Zijlstra <[email protected]>Signed-off-by Paul E. McKenney <[email protected]>[ paulmck: Backported to make patch apply cleanly on older versions. ]Tested-by Marius Hillenbrand <[email protected]>Cc <[email protected]> # 4.12.x - 4.19.x
Now queued up, thanks.
greg k-h