From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: David Woodhouse <dwmw2@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 09:44:20 -0700 [thread overview]
Message-ID: <20180709164420.GA8249@linux.vnet.ibm.com> (raw)
In-Reply-To: <20180709163432.GV3593@linux.vnet.ibm.com>
On Mon, Jul 09, 2018 at 09:34:32AM -0700, Paul E. McKenney wrote:
> On Mon, Jul 09, 2018 at 05:26:32PM +0200, Peter Zijlstra wrote:
> > On Mon, Jul 09, 2018 at 07:29:32AM -0700, Paul E. McKenney wrote:
> > > OK, so here are our options:
> > >
> > > 1. Add the RCU conditional to need_resched(), as David suggests.
> > > Peter has concerns about overhead.
> > >
> > > 2. Create a new need_resched_rcu_qs() that is to be used when
> > > deciding whether or not to do cond_resched(). This would
> > > exact the overhead only where it is needed, but is one more
> > > thing for people to get wrong.
> >
> > Also, with the crypto guys checking need_resched() in asm that won't
> > really work either.
>
> Fair point! Ease of use is a good thing, even within the Linux kernel.
> Or maybe especially within the Linux kernel...
>
> > > 3. Revert my changes to de-emphasize cond_resched_rcu_qs(),
> > > and go back to sprinkling cond_resched_rcu_qs() throughout
> > > the code. This also is one more thing for people to get wrong,
> > > and might well eventually convert all cond_resched() calls to
> > > cond_resched_rcu_qs(), which sure seems like a failure mode to me.
> >
> > 4a. use resched_cpu() more agressive
> > 4b. use the tick to set TIF_NEED_RESCHED when it finds rcu_urgent_qs
> > (avoids the IPI at the 'cost' of a slight delay in processing)
>
> 4b sounds eminently reasonable to me! Something like the (untested,
> probably doesn't even build) patch below?
>
> David, any reason why this wouldn't work? Seems to me that this would
> make need_resched() respond to RCU's need for quiescent states in a
> timely manner without need_resched() having to become heavier weight,
> but figured I should ask.
>
> > 5. make guest mode a quiescent state (like supposedly already done
> > for NOHZ_FULL) (but this would not help the crypto guys).
> >
> > 6. ....
> >
> > ok I ran out of ideas here I think.
> >
> >
> > So for PREEMPT the tick can check preempt_count() == 0 and if so, know
> > it _could_ have rescheduled and advance the qs, right? But since we
> > don't have a preempt count for !PREEMPT_COUNT kernels this doesn't work.
> >
> > And thus we need to invoke actual scheduling events and then through the
> > schedule() callback RCU knows things happened.
> >
> > 4b seems like something worth trying for !PREEMPT kernels I suppose
>
> David is running a !PREEMPT kernel.
>
> For PREEMPT kernels, the patch below results in a quiescent state for
> the CPU, and the forced schedule queues the task. This queuing enables
> later RCU priority boosting (if enabled) once all other CPUs sharing
> the same leaf rcu_node structure have passed through quiescent states.
>
> And yes, for PREEMPT kernels the scheduling-clock interrupt handler
> already checks for a quiescent state using a combination of
> preempt_count() (as you say, but ignoring the hardirq bits because
> we are in an interrupt handler) and current->rcu_read_lock_nesting.
>
> So I believe that this will cover it.
>
> Thoughts?
Updated per Peter's feedback on IRC.
Thanx, Paul
------------------------------------------------------------------------
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 51919985f6cf..ccde5f8aff61 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -2496,6 +2496,12 @@ 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);
+ 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 16:42 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 [this message]
2018-07-09 18:50 ` David Woodhouse
2018-07-09 20:34 ` Paul E. McKenney
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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