linux-kernel - Re: 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: <ZrDDH8XWczouHJdb@localhost.localdomain>
Date: Mon, 5 Aug 2024 14:18:39 +0200
From: Frederic Weisbecker <frederic@...nel.org>
To: Peter Zijlstra <peterz@...radead.org>
Cc: "Paul E. McKenney" <paulmck@...nel.org>,
	Valentin Schneider <vschneid@...hat.com>, rcu@...r.kernel.org,
	linux-kernel@...r.kernel.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: Re: RCU-Task[-Trace] VS EQS (was Re: [PATCH v3 13/25]
 context_tracking, rcu: Rename rcu_dynticks_task*() into rcu_task*())

Le Mon, Aug 05, 2024 at 11:01:22AM +0200, Peter Zijlstra a écrit :
> On Fri, Aug 02, 2024 at 09:32:08PM -0700, Paul E. McKenney wrote:
> 
> > Huh.  One reason for the current smp_call_function_single() in
> > cpuhp_report_idle_dead() was some ARM32 CPUs that shut down caching on
> > their way out.  this made it impossible to use shared-variable-based
> > CPU-dead notification.  I wonder if Arnd's deprecation schedule
> > for ARM32-based platforms will allow us to go back to shared-memory
> > notification, which might make this sort of thing easier.
> 
> All those idle paths should be doing ct_cpuidle_enter(), which includes
> telling RCU the CPU is going idle, no?
> 
> I cleaned all that up a while back.

On every architectures? That would be a good news, as we could then remove
the CONFIG_TASKS_TRACE_RCU_READ_MB dance.

There is still some vulnerable path on CPU hotplug down path once
cpuhp_complete_idle_dead() is executed. The dead CPU may still call a bunch
of general purpose functions (the exit path of smp_call_function_single()
and the exit of cpuhp_report_idle_dead()). And this will become worse once
we remove TASK-RUDE and rely only on rcutree_report_cpu_dead().

Thanks.