* [PATCH RCU -stable] Make need_resched() respond to urgent RCU-QS needs
@ 2018-11-05 2:34 Paul E. McKenney
2018-11-29 12:39 ` Greg KH
0 siblings, 1 reply; 2+ messages in thread
From: Paul E. McKenney @ 2018-11-05 2:34 UTC (permalink / raw)
To: stable
Cc: mhillenb, dwmw2, peterz, josh, rostedt, mathieu.desnoyers,
jiangshanlai, linux-kernel
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.
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 make patch apply cleanly on older versions. ]
Tested-by: Marius Hillenbrand <mhillenb@amazon.de>
Cc: <stable@vger.kernel.org> # 4.12.x - 4.19.x
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 0b760c1369f7..15301ed19da6 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -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();
^ permalink raw reply related [flat|nested] 2+ messages in thread
* Re: [PATCH RCU -stable] Make need_resched() respond to urgent RCU-QS needs
2018-11-05 2:34 [PATCH RCU -stable] Make need_resched() respond to urgent RCU-QS needs Paul E. McKenney
@ 2018-11-29 12:39 ` Greg KH
0 siblings, 0 replies; 2+ messages in thread
From: Greg KH @ 2018-11-29 12:39 UTC (permalink / raw)
To: Paul E. McKenney
Cc: stable, mhillenb, dwmw2, peterz, josh, rostedt,
mathieu.desnoyers, jiangshanlai, linux-kernel
On Sun, Nov 04, 2018 at 06:34:34PM -0800, Paul E. McKenney wrote:
> 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.
>
> 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 make patch apply cleanly on older versions. ]
> Tested-by: Marius Hillenbrand <mhillenb@amazon.de>
> Cc: <stable@vger.kernel.org> # 4.12.x - 4.19.x
>
Now queued up, thanks.
greg k-h
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