linux-kernel - RCU-Task[-Trace] VS EQS (was Re: [PATCH v3 13/25] context_tracking, rcu: Rename rcu_dynticks_task*() into rcu

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Message-ID: <ZqJiDlKtD4wvsv1j@localhost.localdomain>
Date: Thu, 25 Jul 2024 16:32:46 +0200
From: Frederic Weisbecker <frederic@...nel.org>
To: Valentin Schneider <vschneid@...hat.com>,
	"Paul E. McKenney" <paulmck@...nel.org>
Cc: rcu@...r.kernel.org, linux-kernel@...r.kernel.org,
	Peter Zijlstra <peterz@...radead.org>,
	Neeraj Upadhyay <quic_neeraju@...cinc.com>,
	Joel Fernandes <joel@...lfernandes.org>,
	Josh Triplett <josh@...htriplett.org>,
	Boqun Feng <boqun.feng@...il.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Lai Jiangshan <jiangshanlai@...il.com>,
	Zqiang <qiang.zhang1211@...il.com>
Subject: RCU-Task[-Trace] VS EQS (was Re: [PATCH v3 13/25] context_tracking,
 rcu: Rename rcu_dynticks_task*() into rcu_task*())

Le Wed, Jul 24, 2024 at 04:43:13PM +0200, Valentin Schneider a écrit :
> -/* Turn on heavyweight RCU tasks trace readers on idle/user entry. */
> -static __always_inline void rcu_dynticks_task_trace_enter(void)
> +/* Turn on heavyweight RCU tasks trace readers on kernel exit. */
> +static __always_inline void rcu_task_trace_exit(void)

Before I proceed on this last one, a few questions for Paul and others:

1) Why is rcu_dynticks_task_exit() not called while entering in NMI?
   Does that mean that NMIs aren't RCU-Task read side critical sections?

2) Looking further into CONFIG_TASKS_TRACE_RCU_READ_MB=y, it seems to
   allow for uses of rcu_read_[un]lock_trace() while RCU is not watching
   (EQS). Is it really a good idea to support that? Are we aware of any
   such potential usecase?

Thanks!