Re: [RFC] Make need_resched() return true when rcu_urgent_qs requested

Paul E. McKenney
2018-07-11T16:47:41Z

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 <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: 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 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)