From: Valentin Schneider <valentin.schneider@arm.com>
To: paulmck@kernel.org, Frederic Weisbecker <frederic@kernel.org>
Cc: linux-kernel@vger.kernel.org,
linux-arm-kernel@lists.infradead.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>,
Thomas Gleixner <tglx@linutronix.de>,
Steven Rostedt <rostedt@goodmis.org>,
Daniel Bristot de Oliveira <bristot@redhat.com>,
Josh Triplett <josh@joshtriplett.org>,
Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
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>,
Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Subject: Re: [PATCH 2/3] rcu/nocb: Check for migratability rather than pure preemptability
Date: Thu, 29 Jul 2021 11:51:32 +0100 [thread overview]
Message-ID: <87mtq5l7ez.mognet@arm.com> (raw)
In-Reply-To: <20210729010445.GO4397@paulmck-ThinkPad-P17-Gen-1>
On 28/07/21 18:04, Paul E. McKenney wrote:
> On Thu, Jul 29, 2021 at 12:01:37AM +0200, Frederic Weisbecker wrote:
>> On Wed, Jul 28, 2021 at 08:34:14PM +0100, Valentin Schneider wrote:
>> > Now, if the offloaded state was (properly) protected by a local_lock, do
>> > you reckon we could then keep preemption enabled?
>>
>> I guess we could take such a local lock on the update side
>> (rcu_nocb_rdp_offload) and then take it on rcuc kthread/softirqs
>> and maybe other places.
>>
>> But we must make sure that rcu_core() is preempt-safe from a general perspective
>> in the first place. From a quick glance I can't find obvious issues...yet.
>>
>> Paul maybe you can see something?
>
> Let's see...
>
> o Extra context switches in rcu_core() mean extra quiescent
> states. It therefore might be necessary to wrap rcu_core()
> in an rcu_read_lock() / rcu_read_unlock() pair, because
> otherwise an RCU grace period won't wait for rcu_core().
>
> Actually, better have local_bh_disable() imply
> rcu_read_lock() and local_bh_enable() imply rcu_read_unlock().
> But I would hope that this already happened.
It does look like it.
>
> o The rcu_preempt_deferred_qs() check should still be fine,
> unless there is a raw_bh_disable() in -rt.
>
> o The set_tsk_need_resched() and set_preempt_need_resched()
> might preempt immediately. I cannot think of a problem
> with that, but careful testing is clearly in order.
>
> o The values checked by rcu_check_quiescent_state() could now
> change while this function is running. I don't immediately
> see a problematic sequence of events, but here be dragons.
> I therefore suggest disabling preemption across this function.
> Or if that is impossible, taking a very careful look at the
> proposed expansion of the state space of this function.
>
> o I don't see any new races in the grace-period/callback check.
> New callbacks can appear in interrupt handlers, after all.
>
> o The rcu_check_gp_start_stall() function looks similarly
> unproblematic.
>
> o Callback invocation can now be preempted, but then again it
> recently started being concurrent, so this should be no
> added risk over offloading/de-offloading.
>
> o I don't see any problem with do_nocb_deferred_wakeup().
>
> o The CONFIG_RCU_STRICT_GRACE_PERIOD check should not be
> impacted.
>
> So some adjustments might be needed, but I don't see a need for
> major surgery.
>
> This of course might be a failure of imagination on my part, so it
> wouldn't hurt to double-check my observations.
>
I'll go poke around, thank you both!
next prev parent reply other threads:[~2021-07-29 10:51 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-07-21 11:51 [PATCH 0/3] sched: migrate_disable() vs per-CPU access safety checks Valentin Schneider
2021-07-21 11:51 ` [PATCH 1/3] sched: Introduce is_pcpu_safe() Valentin Schneider
2021-07-27 16:23 ` Paul E. McKenney
2021-07-21 11:51 ` [PATCH 2/3] rcu/nocb: Check for migratability rather than pure preemptability Valentin Schneider
2021-07-27 16:24 ` Paul E. McKenney
2021-07-27 23:08 ` Frederic Weisbecker
2021-07-28 19:34 ` Valentin Schneider
2021-07-28 22:01 ` Frederic Weisbecker
2021-07-29 1:04 ` Paul E. McKenney
2021-07-29 10:51 ` Valentin Schneider [this message]
2021-07-21 11:51 ` [PATCH 3/3] arm64: mm: Make arch_faults_on_old_pte() check for migratability Valentin Schneider
2021-07-27 19:45 ` [PATCH 0/3] sched: migrate_disable() vs per-CPU access safety checks Thomas Gleixner
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