linux-kernel - Re: [PATCH 3/4] workqueue: Tag bound workers with KTHREAD_IS_PER

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Date:   Wed, 13 Jan 2021 10:59:45 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Valentin Schneider <valentin.schneider@....com>
Cc:     Lai Jiangshan <jiangshanlai@...il.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>, Qian Cai <cai@...hat.com>,
        Vincent Donnefort <vincent.donnefort@....com>,
        Dexuan Cui <decui@...rosoft.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Steven Rostedt <rostedt@...dmis.org>, Tejun Heo <tj@...nel.org>
Subject: Re: [PATCH 3/4] workqueue: Tag bound workers with KTHREAD_IS_PER_CPU

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