Re: [PATCH 3/4] workqueue: Tag bound workers with KTHREAD_IS_PER_CPU

From: "Paul E. McKenney" <paulmck@kernel.org>
To: Valentin Schneider <valentin.schneider@arm.com>
Cc: Lai Jiangshan <jiangshanlai@gmail.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Ingo Molnar <mingo@kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	LKML <linux-kernel@vger.kernel.org>, Qian Cai <cai@redhat.com>,
	Vincent Donnefort <vincent.donnefort@arm.com>,
	Dexuan Cui <decui@microsoft.com>,
	Vincent Guittot <vincent.guittot@linaro.org>,
	Steven Rostedt <rostedt@goodmis.org>, Tejun Heo <tj@kernel.org>
Subject: Re: [PATCH 3/4] workqueue: Tag bound workers with KTHREAD_IS_PER_CPU
Date: Wed, 13 Jan 2021 10:59:45 -0800	[thread overview]
Message-ID: <20210113185945.GZ2743@paulmck-ThinkPad-P72> (raw)
In-Reply-To: <jhjpn28zngy.mognet@arm.com>

On Wed, Jan 13, 2021 at 06:43:57PM +0000, Valentin Schneider wrote:
> On 13/01/21 09:52, Paul E. McKenney wrote:
> > On Wed, Jan 13, 2021 at 02:16:10PM +0000, Valentin Schneider wrote:
> >> You might be right; at this point we would still have BALANCE_PUSH set,
> >> so something like the below could happen
> >>
> >>   rebind_workers()
> >>     set_cpus_allowed_ptr()
> >>       affine_move_task()
> >>         task_running() => stop_one_cpu()
> >>
> >>   ... // Stopper migrates the kworker here in the meantime
> >>
> >>   switch_to(<pcpu kworker>) // Both cpuhp thread and kworker should be enqueued
> >>                             // here, so one or the other could be picked
> >>   balance_switch()
> >>     balance_push()
> >>     ^-- no KTHREAD_IS_PER_CPU !
> >>
> >> This should however trigger the WARN_ON_ONCE() in kthread_set_per_cpu()
> >> *before* the one in process_one_work(), which I haven't seen in Paul's
> >> mails.
> >
> > The 56 instances of one-hour SRCU-P scenarios hit the WARN_ON_ONCE()
> > in process_one_work() once, but there is no sign of a WARN_ON_ONCE()
> > from kthread_set_per_cpu().
> 
> This does make me doubt the above :/ At the same time, the
> process_one_work() warning hinges on POOL_DISASSOCIATED being unset,
> which implies having gone through rebind_workers(), which implies
> kthread_set_per_cpu(), which implies me being quite confused...
> 
> > But to your point, this does appear to be
> > a rather low-probability race condition, once per some tens of hours
> > of SRCU-P.
> >
> > Is there a more focused check for the race condition above?
> 
> Not that I'm aware of. I'm thinking that if the pcpu kworker were an RT
> task, then this would guarantee it would get picked in favor of the cpuhp
> thread upon switching out of the stopper, but that still requires the
> kworker running on some CPU (for some reason) during rebind_workers().

Well, I did use the rcutree.softirq=0 boot parameter, which creates
per-CPU rcuc kthreads to do what RCU_SOFTIRQ normally does.  But these
rcuc kthreads use the normal park/unpark discipline, so should be safe,
for some value of "should".

							Thanx, Paul