From: Frederic Weisbecker <frederic@kernel.org>
To: "Paul E. McKenney" <paulmck@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>,
Valentin Schneider <valentin.schneider@arm.com>,
linux-kernel@vger.kernel.org,
linux-arm-kernel@lists.infradead.org, rcu@vger.kernel.org,
linux-rt-users@vger.kernel.org,
Catalin Marinas <catalin.marinas@arm.com>,
Will Deacon <will@kernel.org>, Ingo Molnar <mingo@kernel.org>,
Peter Zijlstra <peterz@infradead.org>,
Steven Rostedt <rostedt@goodmis.org>,
Daniel Bristot de Oliveira <bristot@redhat.com>,
Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
Josh Triplett <josh@joshtriplett.org>,
Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
Davidlohr Bueso <dave@stgolabs.net>,
Lai Jiangshan <jiangshanlai@gmail.com>,
Joel Fernandes <joel@joelfernandes.org>,
Anshuman Khandual <anshuman.khandual@arm.com>,
Vincenzo Frascino <vincenzo.frascino@arm.com>,
Steven Price <steven.price@arm.com>,
Ard Biesheuvel <ardb@kernel.org>,
Boqun Feng <boqun.feng@gmail.com>, Mike Galbraith <efault@gmx.de>
Subject: Re: rcu/tree: Protect rcu_rdp_is_offloaded() invocations on RT
Date: Wed, 22 Sep 2021 13:31:17 +0200 [thread overview]
Message-ID: <20210922113117.GB106513@lothringen> (raw)
In-Reply-To: <20210922021837.GV880162@paulmck-ThinkPad-P17-Gen-1>
On Tue, Sep 21, 2021 at 07:18:37PM -0700, Paul E. McKenney wrote:
> On Wed, Sep 22, 2021 at 01:36:27AM +0200, Frederic Weisbecker wrote:
> > Doing the local_irq_save() before checking that the segcblist is offloaded
> > protect that state from being changed (provided we lock the local rdp). Then we
> > can safely manipulate cblist, whether locked or unlocked.
> >
> > 2) The actual call to rcu_do_batch(). If we are preempted between
> > rcu_segcblist_completely_offloaded() and rcu_do_batch() with a deoffload in
> > the middle, we miss the callback invocation. Invoking rcu_core by the end of
> > deoffloading process should solve that.
>
> Maybe invoke rcu_core() at that point? My concern is that there might
> be an extended time between the missed rcu_do_batch() and the end of
> the deoffloading process.
Agreed!
>
> > > Reported-by: Valentin Schneider <valentin.schneider@arm.com>
> > > Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> > > ---
> > > kernel/rcu/tree.c | 7 ++++---
> > > 1 file changed, 4 insertions(+), 3 deletions(-)
> > >
> > > --- a/kernel/rcu/tree.c
> > > +++ b/kernel/rcu/tree.c
> > > @@ -2278,13 +2278,13 @@ rcu_report_qs_rdp(struct rcu_data *rdp)
> > > {
> > > unsigned long flags;
> > > unsigned long mask;
> > > - bool needwake = false;
> > > - const bool offloaded = rcu_rdp_is_offloaded(rdp);
> > > + bool offloaded, needwake = false;
> > > struct rcu_node *rnp;
> > >
> > > WARN_ON_ONCE(rdp->cpu != smp_processor_id());
> > > rnp = rdp->mynode;
> > > raw_spin_lock_irqsave_rcu_node(rnp, flags);
> > > + offloaded = rcu_rdp_is_offloaded(rdp);
> > > if (rdp->cpu_no_qs.b.norm || rdp->gp_seq != rnp->gp_seq ||
> > > rdp->gpwrap) {
> >
> > BTW Paul, if we happen to switch to non-NOCB (deoffload) some time after
> > rcu_report_qs_rdp(), it's possible that the rcu_accelerate_cbs()
> > that was supposed to be handled by nocb kthreads on behalf of
> > rcu_core() -> rcu_report_qs_rdp() would not happen. At least not until
> > we invoke rcu_core() again. Not sure how much harm that could cause.
>
> Again, can we just invoke rcu_core() as soon as this is noticed?
Right. So I'm going to do things a bit differently. I'm going to add
a new segcblist state flag so that during the deoffloading process,
the first very step is an invoke_rcu_core() on the target after setting a
flag that requires handling all this things: accelerate/do_batch, etc...
Then will remain the "do we still have pending callbacks after do_batch?"
in which case we'll need to invoke the rcu_core again as long as we are in
the middle of deoffloading.
Ok, now to write the patches.
next prev parent reply other threads:[~2021-09-22 11:31 UTC|newest]
Thread overview: 51+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-08-11 20:13 [PATCH v3 0/4] rcu, arm64: PREEMPT_RT fixlets Valentin Schneider
2021-08-11 20:13 ` [PATCH v3 1/4] rcutorture: Don't disable softirqs with preemption disabled when PREEMPT_RT Valentin Schneider
2021-08-12 16:47 ` Paul E. McKenney
2021-08-17 12:13 ` Sebastian Andrzej Siewior
2021-08-17 13:17 ` Sebastian Andrzej Siewior
2021-08-17 14:40 ` [PATCH] rcutorture: Avoid problematic critical section nesting on RT Sebastian Andrzej Siewior
2021-08-18 22:46 ` Paul E. McKenney
2021-08-19 15:35 ` Sebastian Andrzej Siewior
2021-08-19 15:39 ` Sebastian Andrzej Siewior
2021-08-19 15:47 ` Sebastian Andrzej Siewior
2021-08-19 18:20 ` Paul E. McKenney
2021-08-19 18:45 ` Sebastian Andrzej Siewior
2021-08-20 4:11 ` Scott Wood
2021-08-20 7:11 ` Sebastian Andrzej Siewior
2021-08-20 7:42 ` [PATCH v2] rcutorture: Avoid problematic critical section nesting on PREEMPT_RT Sebastian Andrzej Siewior
2021-08-20 22:10 ` Paul E. McKenney
2021-08-20 3:23 ` [PATCH] rcutorture: Avoid problematic critical section nesting on RT Scott Wood
2021-08-20 6:54 ` Sebastian Andrzej Siewior
2021-08-11 20:13 ` [PATCH v3 2/4] sched: Introduce migratable() Valentin Schneider
2021-08-17 14:43 ` Sebastian Andrzej Siewior
2021-08-22 17:31 ` Valentin Schneider
2021-08-17 17:09 ` Sebastian Andrzej Siewior
2021-08-17 19:30 ` Phil Auld
2021-08-22 18:14 ` Valentin Schneider
2022-01-26 16:56 ` Sebastian Andrzej Siewior
2022-01-26 18:10 ` Valentin Schneider
2022-01-27 10:07 ` Sebastian Andrzej Siewior
2022-01-27 18:23 ` Valentin Schneider
2022-01-27 19:27 ` Valentin Schneider
2022-02-04 9:24 ` Sebastian Andrzej Siewior
2021-08-11 20:13 ` [PATCH v3 3/4] rcu/nocb: Protect NOCB state via local_lock() under PREEMPT_RT Valentin Schneider
2021-08-13 0:20 ` Paul E. McKenney
2021-08-13 18:48 ` Valentin Schneider
2021-08-17 15:36 ` Sebastian Andrzej Siewior
2021-08-22 18:15 ` Valentin Schneider
2021-09-21 14:05 ` Thomas Gleixner
2021-09-21 21:12 ` rcu/tree: Protect rcu_rdp_is_offloaded() invocations on RT Thomas Gleixner
2021-09-21 23:36 ` Frederic Weisbecker
2021-09-22 2:18 ` Paul E. McKenney
2021-09-22 11:31 ` Frederic Weisbecker [this message]
2021-09-21 23:45 ` Frederic Weisbecker
2021-09-22 6:32 ` Sebastian Andrzej Siewior
2021-09-22 11:10 ` Frederic Weisbecker
2021-09-22 11:27 ` Sebastian Andrzej Siewior
2021-09-22 11:38 ` Frederic Weisbecker
2021-09-22 13:02 ` Sebastian Andrzej Siewior
2021-09-23 10:02 ` Frederic Weisbecker
2021-09-30 9:00 ` Valentin Schneider
2021-09-30 10:53 ` Frederic Weisbecker
2021-09-30 13:22 ` Valentin Schneider
2021-08-11 20:13 ` [PATCH v3 4/4] arm64: mm: Make arch_faults_on_old_pte() check for migratability Valentin Schneider
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