From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: David Woodhouse <dwmw2@infradead.org>
Cc: Peter Zijlstra <peterz@infradead.org>,
mhillenb@amazon.de, linux-kernel <linux-kernel@vger.kernel.org>
Subject: Re: [RFC] Make need_resched() return true when rcu_urgent_qs requested
Date: Mon, 9 Jul 2018 13:34:41 -0700 [thread overview]
Message-ID: <20180709203441.GE3593@linux.vnet.ibm.com> (raw)
In-Reply-To: <1531162254.26547.3.camel@infradead.org>
On Mon, Jul 09, 2018 at 07:50:54PM +0100, David Woodhouse wrote:
>
>
> On Mon, 2018-07-09 at 09:34 -0700, Paul E. McKenney wrote:
> > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > index 51919985f6cf..33b0a1ec0536 100644
> > --- a/kernel/rcu/tree.c
> > +++ b/kernel/rcu/tree.c
> > @@ -2496,6 +2496,10 @@ void rcu_check_callbacks(int user)
> > {
> > trace_rcu_utilization(TPS("Start scheduler-tick"));
> > raw_cpu_inc(rcu_data.ticks_this_gp);
> > + if (smp_load_acquire(this_cpu_ptr(&rcu_dynticks.rcu_urgent_qs)) &&
> > + !is_idle_task(current))
> > + set_tsk_need_resched(current);
>
> OK, so this will make KVM (and various other code) see that
> need_resched() is true, and they'll call cond_resched() or something
> else that might not actually schedule another task, but will at least
> end up in rcu_all_qs()...
>
> > + __this_cpu_write(rcu_dynticks.rcu_urgent_qs, false);
>
> ... which bails out immediately and does nothing, because that's set to
> false?
>
> Am I missing something?
If this is the idle task, RCU will detect that as a quiescent state via
its dyntick-idle mechanism. In which case, there is no point in leaving
.rcu_urgent_qs being true.
If this is not the idle task, the scheduler will invoke
rcu_note_context_switch(), which will in turn invoke rcu_sched_qs(),
rcu_preempt_qs(), or rcu_qs(), depending on kernel version and
configuration. This will happen independently of .rcu_urgent_qs, so it
is OK to set .rcu_urgent_qs to false. And doing so reduces the overhead
of the next cond_resched().
I may end up using rcu_is_cpu_rrupt_from_idle() instead of is_idle_task()
at some point, but the former eases backporting. And the only difference
is if someone has a long loop within an _rcuidle tracepoint used in the
idle loop, and where that loop check need_resched(). Which currently
seems to be the empty set.
And I should treat interruption of a usermode task the same as that of an
idle task. In the PREEMPT case, ->rcu_read_lock_nesting better be zero
(lockdep would have complained), so the quiescent state will be reported.
In the !PREEMPT case, user=1 directly causes reporting of a quiescent
state.
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.)
2. A cond_resched() will cause rcu_all_qs() to be invoked, which
will record a quiescent state and clear .rcu_urgent_qs.
3. With the patch below, a scheduling-clock interrupt of a
non-idle non-userspace task will force a reschedule, which
will result in #1 above happening.
However, I should avoid setting .rcu_urgent_qs to false when it is
already false, shouldn't I?
So how about the following instead?
I am doing some light testing and will let you know how that goes.
Thanx, Paul
------------------------------------------------------------------------
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 51919985f6cf..c3b688c7127a 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 already is a quiescent state. */
+ 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();
next prev parent reply other threads:[~2018-07-09 20:32 UTC|newest]
Thread overview: 93+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-07-06 14:53 [RFC] Make need_resched() return true when rcu_urgent_qs requested David Woodhouse
2018-07-06 16:29 ` Peter Zijlstra
2018-07-06 17:11 ` Paul E. McKenney
2018-07-06 17:14 ` David Woodhouse
2018-07-06 21:12 ` Paul E. McKenney
2018-07-09 8:58 ` Peter Zijlstra
2018-07-09 8:53 ` Peter Zijlstra
2018-07-09 9:18 ` David Woodhouse
2018-07-09 10:44 ` Peter Zijlstra
2018-07-09 10:56 ` David Woodhouse
2018-07-09 11:06 ` Peter Zijlstra
2018-07-09 11:12 ` David Woodhouse
2018-07-09 11:31 ` Peter Zijlstra
2018-07-09 12:34 ` Paul E. McKenney
2018-07-09 12:47 ` David Woodhouse
2018-07-09 14:30 ` Paul E. McKenney
2018-07-09 12:55 ` Peter Zijlstra
2018-07-09 12:57 ` David Woodhouse
2018-07-09 13:02 ` Peter Zijlstra
2018-07-09 14:29 ` Paul E. McKenney
2018-07-09 14:43 ` Peter Zijlstra
2018-07-09 14:54 ` Paul E. McKenney
2018-07-09 15:26 ` Peter Zijlstra
2018-07-09 16:34 ` Paul E. McKenney
2018-07-09 16:44 ` Paul E. McKenney
2018-07-09 18:50 ` David Woodhouse
2018-07-09 20:34 ` Paul E. McKenney [this message]
2018-07-09 20:35 ` David Woodhouse
2018-07-09 20:42 ` Paul E. McKenney
2018-07-09 20:45 ` David Woodhouse
2018-07-09 21:05 ` Paul E. McKenney
2018-07-09 22:08 ` Paul E. McKenney
2018-07-11 10:57 ` David Woodhouse
2018-07-11 12:51 ` Paul E. McKenney
2018-07-11 12:58 ` David Woodhouse
2018-07-11 14:25 ` Paul E. McKenney
2018-07-11 14:23 ` David Woodhouse
2018-07-11 14:43 ` Paul E. McKenney
2018-07-11 16:49 ` Paul E. McKenney
2018-07-11 17:03 ` David Woodhouse
2018-07-11 17:48 ` Paul E. McKenney
2018-07-11 18:01 ` [PATCH v2] kvm/x86: Inform RCU of quiescent state when entering guest mode David Woodhouse
2018-07-11 18:20 ` Paul E. McKenney
2018-07-11 18:36 ` Paul E. McKenney
2018-07-11 18:39 ` Christian Borntraeger
2018-07-11 20:27 ` Paul E. McKenney
2018-07-11 20:54 ` David Woodhouse
2018-07-11 21:09 ` Paul E. McKenney
2018-07-11 21:11 ` Christian Borntraeger
2018-07-11 21:32 ` Paul E. McKenney
2018-07-11 21:39 ` Christian Borntraeger
2018-07-11 23:47 ` Paul E. McKenney
2018-07-12 8:31 ` David Woodhouse
2018-07-12 11:00 ` Christian Borntraeger
2018-07-12 11:10 ` David Woodhouse
2018-07-12 11:58 ` Christian Borntraeger
2018-07-12 12:04 ` Christian Borntraeger
2018-07-11 23:37 ` Paul E. McKenney
2018-07-12 2:15 ` Paul E. McKenney
2018-07-12 6:21 ` Christian Borntraeger
2018-07-12 9:52 ` David Woodhouse
2018-07-11 18:31 ` [RFC] Make need_resched() return true when rcu_urgent_qs requested Christian Borntraeger
2018-07-11 20:17 ` Paul E. McKenney
2018-07-11 20:19 ` David Woodhouse
2018-07-11 21:08 ` Paul E. McKenney
2018-07-12 12:00 ` David Woodhouse
2018-07-12 12:53 ` Paul E. McKenney
2018-07-12 16:17 ` Paul E. McKenney
2018-07-16 15:40 ` Paul E. McKenney
2018-07-17 8:19 ` David Woodhouse
2018-07-17 12:56 ` Paul E. McKenney
2018-07-18 15:36 ` Paul E. McKenney
2018-07-18 16:01 ` David Woodhouse
2018-07-18 16:37 ` Paul E. McKenney
2018-07-18 19:41 ` David Woodhouse
2018-07-18 20:17 ` Paul E. McKenney
2018-07-19 0:26 ` Frederic Weisbecker
2018-07-19 6:45 ` Christian Borntraeger
2018-07-19 7:20 ` David Woodhouse
2018-07-19 10:23 ` Christian Borntraeger
2018-07-19 12:55 ` Paul E. McKenney
2018-07-19 13:14 ` Frederic Weisbecker
2018-07-19 13:36 ` David Woodhouse
2018-07-19 17:09 ` Paul E. McKenney
2018-07-23 8:08 ` David Woodhouse
2018-07-23 12:22 ` Paul E. McKenney
2018-07-19 0:32 ` Frederic Weisbecker
2018-07-19 3:11 ` Paul E. McKenney
2018-07-19 6:16 ` David Woodhouse
2018-07-19 13:17 ` Frederic Weisbecker
2018-07-19 13:15 ` Frederic Weisbecker
2018-07-10 9:24 ` Peter Zijlstra
2018-07-10 16:26 ` Paul E. McKenney
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