Messages in this thread Patch in this message | | | Date | Mon, 9 Jul 2018 14:05:32 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [RFC] Make need_resched() return true when rcu_urgent_qs requested |
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On Mon, Jul 09, 2018 at 09:45:45PM +0100, David Woodhouse wrote: > On Mon, 2018-07-09 at 13:42 -0700, Paul E. McKenney wrote: > > On Mon, Jul 09, 2018 at 09:35:38PM +0100, David Woodhouse wrote: > > > > > > > > > On Mon, 2018-07-09 at 13:34 -0700, Paul E. McKenney wrote: > > > > > > > > So here are the possible code paths when .rcu_urgent_qs is set to true: > > > > > > > > 1. A context switch will record the quiescent state and clear > > > > .rcu_urgent_qs. (The failure to do the clearing in current -rcu > > > > for PREEMPT builds is a performance bug that I need to fix.) > > > > > > What if there's nothing else runnable and there is no actual context > > > switch? > > > > The scheduler invokes rcu_note_context_switch() before looking to see > > if there really will or won't be a context switch. > > > > I am sure that Peter will correct me if I am confused on this point. ;-) > > Ah, OK. Yes, that looks correct. Thanks.
Here is hoping!
> I'll give your patch a spin tomorrow, unless Marius beats me to it.
Please see below for the version that I eventually queued. Should Marius have a Reported-by? If so, please tell me his full name so I can add that.
Thanx, Paul
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commit e1e91fd0796dc544a77ddfaa5afbfc1f5fb42ecd Author: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Date: Mon Jul 9 13:47:30 2018 -0700
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: David Woodhouse <dwmw2@infradead.org> Suggested-by: Peter Zijlstra <peterz@infradead.org> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c index 51919985f6cf..ea756bb64eb3 100644 --- a/kernel/rcu/tree.c +++ b/kernel/rcu/tree.c @@ -2496,6 +2496,15 @@ void rcu_check_callbacks(int user) { trace_rcu_utilization(TPS("Start scheduler-tick")); raw_cpu_inc(rcu_data.ticks_this_gp); + /* 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 (!is_idle_task(current) && !user) { + set_tsk_need_resched(current); + set_preempt_need_resched(); + } + __this_cpu_write(rcu_dynticks.rcu_urgent_qs, false); + } rcu_flavor_check_callbacks(user); if (rcu_pending()) invoke_rcu_core();
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