On Wed, Jul 11, 2018 at 07:43:03AM -0700, Paul E. McKenney wrote: > On Wed, Jul 11, 2018 at 03:23:45PM +0100, David Woodhouse wrote: > > > > > > On Mon, 2018-07-09 at 15:08 -0700, Paul E. McKenney wrote: > > > index f9c0ca2ccf0c..3350ece366ab 100644 > > > --- a/kernel/rcu/tree.c > > > +++ b/kernel/rcu/tree.c > > > @@ -2839,6 +2839,15 @@ 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) { > > > > if (idle && !user) seems tautological and... illogical. > > > > If I make it 'if (!rcu_is_cpu_rrput_from_idle() && !user)' it seems to > > work better. Ripping out my debugging printks now to check that's still > > true... > > Right you are! I will step away for a bit to put a paper bag over > my head... > > > (Also, isn't userspace execution only a quiescent state if NO_HZ_FULL?) > > Userspace execution is a quiescent state in all cases. However, you > are quite right that NO_HZ_FULL makes a difference, namely, it allows > one CPU to reliably determine whether or not some other CPU is > currently executing either in userspace or in idle. > > Without NO_HZ_FULL, CPUs can only detect their own userspace execution. > Which is what is happening here because rcu_check_callbacks() is being > invoked from the scheduling-clock interrupt, which is where the "user" > parameter comes from. > > So the above code can reliably detect the usermode-execution quiescent > state because it is always running on the CPU in question.
And here is an updated v4.15 patch with Marius's Reported-by and David's fix to my lost exclamation point. Thanx, Paul ------------------------------------------------------------------------ commit 83c4beae36f2a2b38c1a0fa80538af7ce2477823 Author: Paul E. McKenney <paul...@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: Marius Hillenbrand <mhill...@amazon.de> Reported-by: David Woodhouse <dw...@infradead.org> Suggested-by: Peter Zijlstra <pet...@infradead.org> Signed-off-by: Paul E. McKenney <paul...@linux.vnet.ibm.com> [ paulmck: Backported to v4.15. Probably applies elsewhere. ] diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c index f9c0ca2ccf0c..de2f91cb2a0c 100644 --- a/kernel/rcu/tree.c +++ b/kernel/rcu/tree.c @@ -2839,6 +2839,15 @@ 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(); if (user)