From: Thomas Gleixner <tglx@linutronix.de>
To: Joel Fernandes <joel@joelfernandes.org>
Cc: "Paul E. McKenney" <paulmck@kernel.org>,
LKML <linux-kernel@vger.kernel.org>,
rcu@vger.kernel.org, Andrew Lutomirski <luto@kernel.org>,
X86 ML <x86@kernel.org>,
Frederic Weisbecker <frederic@kernel.org>,
Steven Rostedt <rostedt@goodmis.org>,
Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
Will Deacon <will@kernel.org>,
Peter Zijlstra <peterz@infradead.org>
Subject: Re: [PATCH x86/entry: Force rcu_irq_enter() when in idle task
Date: Tue, 16 Jun 2020 10:40:04 +0200 [thread overview]
Message-ID: <871rmfcskb.fsf@nanos.tec.linutronix.de> (raw)
In-Reply-To: <20200615201647.GC12037@google.com>
Joel,
Joel Fernandes <joel@joelfernandes.org> writes:
> On Fri, Jun 12, 2020 at 03:55:00PM +0200, Thomas Gleixner wrote:
>> This is also correct vs. user mode entries in NOHZ full scenarios because
>> user mode entries bring RCU out of EQS and force the RCU irq nesting state
>
> I had to re-read this sentence a couple of times. The 'user mode entries'
> sounds like 'entry into user mode'. It would be good to reword it to 'IRQ
> entry in user mode'.
:)
> My knowledge predates the rcu-watching reworks so apologies on the below
> question but I still didn't fully follow why when the idle task behaves
> differently from being in user mode. Even with user mode we should have:
>
> <user mode> (in EQS)
> <irq entry> <---------- exits EQS so now rcu is watching
> <softirq entry in the exit path>
> <timer tick irq entry> <-- the buggy !watching logic prevents rcu_irq_enter
> -> report QS since tick thinks it is first level.
>
> Is there a subtlety here I'm missing? I checked the RCU code and I did not
> see anywhere that rcu_user_enter() makes it behave differently. Both
> rcu_user_enter() and rcu_idle_enter() call rcu_eqs_enter().
The interrupt hit user mode entry does:
idtentry_enter_cond_rcu()
if (user_mode(regs))
enter_from_user_mode()
user_exit_irqoff()
__context_tracking_exit(CONTEXT_USER)
rcu_user_exit()
rcu_eqs_exit(1)
...
WRITE_ONCE(rdp->dynticks_nmi_nesting,
DYNTICK_IRQ_NONIDLE);
i.e. for interrupts which enter from user mode we are not invoking
rcu_irq_enter() at all.
The return from interrupt path has nothing to do with that because
that's an entry in kernel mode.
Thanks,
tglx
next prev parent reply other threads:[~2020-06-16 8:40 UTC|newest]
Thread overview: 21+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-06-11 23:53 [PATCH RFC] x86/entry: Ask RCU if it needs rcu_irq_{enter,exit}() Paul E. McKenney
2020-06-11 23:54 ` Paul E. McKenney
2020-06-12 5:30 ` Andy Lutomirski
2020-06-12 12:40 ` Thomas Gleixner
2020-06-12 13:55 ` [PATCH x86/entry: Force rcu_irq_enter() when in idle task Thomas Gleixner
2020-06-12 14:26 ` Frederic Weisbecker
2020-06-12 14:47 ` Thomas Gleixner
2020-06-12 15:32 ` Andy Lutomirski
2020-06-12 17:49 ` Paul E. McKenney
2020-06-12 19:19 ` Paul E. McKenney
2020-06-12 19:25 ` Thomas Gleixner
2020-06-12 19:28 ` Andy Lutomirski
2020-06-12 19:34 ` Thomas Gleixner
2020-06-12 21:56 ` Paul E. McKenney
2020-06-12 19:50 ` [tip: x86/entry] " tip-bot2 for Thomas Gleixner
2020-06-15 20:16 ` [PATCH " Joel Fernandes
2020-06-16 8:40 ` Thomas Gleixner [this message]
2020-06-16 14:30 ` Joel Fernandes
2020-06-16 16:52 ` Andy Lutomirski
2020-06-12 9:27 ` [PATCH RFC] x86/entry: Ask RCU if it needs rcu_irq_{enter,exit}() Thomas Gleixner
2020-06-12 13:57 ` Paul E. McKenney
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