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From: "Paul E. McKenney" <paulmck@linux.ibm.com>
To: Byungchul Park <max.byungchul.park@gmail.com>
Cc: Joel Fernandes <joel@joelfernandes.org>,
	Byungchul Park <byungchul.park@lge.com>,
	rcu <rcu@vger.kernel.org>, LKML <linux-kernel@vger.kernel.org>,
	kernel-team@lge.com
Subject: Re: [PATCH] rcu: Make jiffies_till_sched_qs writable
Date: Fri, 19 Jul 2019 12:57:28 -0700	[thread overview]
Message-ID: <20190719195728.GF14271@linux.ibm.com> (raw)
In-Reply-To: <CANrsvRM7ehvqcPtKMV7RyRCiXwe_R_TsLZiNtxBPY_qnSg2LNQ@mail.gmail.com>

On Fri, Jul 19, 2019 at 06:57:58PM +0900, Byungchul Park wrote:
> On Fri, Jul 19, 2019 at 4:43 PM Paul E. McKenney <paulmck@linux.ibm.com> wrote:
> >
> > On Thu, Jul 18, 2019 at 08:52:52PM -0400, Joel Fernandes wrote:
> > > On Thu, Jul 18, 2019 at 8:40 PM Byungchul Park <byungchul.park@lge.com> wrote:
> > > [snip]
> > > > > - There is a bug in the CPU stopper machinery itself preventing it
> > > > > from scheduling the stopper on Y. Even though Y is not holding up the
> > > > > grace period.
> > > >
> > > > Or any thread on Y is busy with preemption/irq disabled preventing the
> > > > stopper from being scheduled on Y.
> > > >
> > > > Or something is stuck in ttwu() to wake up the stopper on Y due to any
> > > > scheduler locks such as pi_lock or rq->lock or something.
> > > >
> > > > I think what you mentioned can happen easily.
> > > >
> > > > Basically we would need information about preemption/irq disabled
> > > > sections on Y and scheduler's current activity on every cpu at that time.
> > >
> > > I think all that's needed is an NMI backtrace on all CPUs. An ARM we
> > > don't have NMI solutions and only IPI or interrupt based backtrace
> > > works which should at least catch and the preempt disable and softirq
> > > disable cases.
> >
> > True, though people with systems having hundreds of CPUs might not
> > thank you for forcing an NMI backtrace on each of them.  Is it possible
> > to NMI only the ones that are holding up the CPU stopper?
> 
> What a good idea! I think it's possible!
> 
> But we need to think about the case NMI doesn't work when the
> holding-up was caused by IRQ disabled.
> 
> Though it's just around the corner of weekend, I will keep thinking
> on it during weekend!

Very good!

							Thanx, Paul

> Thanks,
> Byungchul
> 
> >                                                         Thanx, Paul
> >
> > > But yeah I don't see why just the stacks of those CPUs that are
> > > blocking the CPU X would not suffice for the trivial cases where a
> > > piece of misbehaving code disable interrupts / preemption and
> > > prevented the stopper thread from executing.
> > >
> > > May be once the test case is ready (no rush!) , then it will be more
> > > clear what can help.
> > >
> > > J.
> > >
> 
> 
> 
> -- 
> Thanks,
> Byungchul
> 


  reply	other threads:[~2019-07-19 19:57 UTC|newest]

Thread overview: 53+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-07-08  6:00 [PATCH] rcu: Make jiffies_till_sched_qs writable Byungchul Park
2019-07-08 12:50 ` Paul E. McKenney
2019-07-08 13:03   ` Joel Fernandes
2019-07-08 13:19     ` Paul E. McKenney
2019-07-08 14:15       ` Joel Fernandes
2019-07-09  6:05       ` Byungchul Park
2019-07-09 12:43         ` Paul E. McKenney
2019-07-09  5:58     ` Byungchul Park
2019-07-09  6:45       ` Byungchul Park
2019-07-09 12:41       ` Paul E. McKenney
2019-07-10  1:20         ` Byungchul Park
2019-07-11 12:30           ` Paul E. McKenney
2019-07-11 13:08             ` Joel Fernandes
2019-07-11 15:02               ` Paul E. McKenney
2019-07-11 16:48                 ` Joel Fernandes
2019-07-11 19:58                   ` Joel Fernandes
2019-07-12  6:32                     ` Byungchul Park
2019-07-12 12:51                       ` Joel Fernandes
2019-07-12 13:02                         ` Paul E. McKenney
2019-07-12 13:43                           ` Joel Fernandes
2019-07-12 14:53                             ` Paul E. McKenney
2019-07-13  8:47                         ` Byungchul Park
2019-07-13 14:20                           ` Joel Fernandes
2019-07-13 15:13                             ` Paul E. McKenney
2019-07-13 15:42                               ` Joel Fernandes
2019-07-13 17:41                                 ` Paul E. McKenney
2019-07-14 13:39                                   ` Byungchul Park
2019-07-14 13:56                                     ` Paul E. McKenney
2019-07-15 17:39                                       ` Joel Fernandes
2019-07-15 20:09                                         ` Paul E. McKenney
2019-07-18 16:14                                   ` Joel Fernandes
2019-07-18 16:15                                     ` Joel Fernandes
2019-07-18 21:34                                     ` Paul E. McKenney
2019-07-19  0:48                                       ` Joel Fernandes
2019-07-19  0:54                                       ` Byungchul Park
2019-07-19  0:39                                     ` Byungchul Park
2019-07-19  0:52                                       ` Joel Fernandes
2019-07-19  1:10                                         ` Byungchul Park
2019-07-19  7:43                                         ` Paul E. McKenney
2019-07-19  9:57                                           ` Byungchul Park
2019-07-19 19:57                                             ` Paul E. McKenney [this message]
2019-07-19 20:33                                               ` Joel Fernandes
2019-07-23 11:05                                                 ` Byungchul Park
2019-07-23 13:47                                                   ` Paul E. McKenney
2019-07-23 16:54                                                     ` Paul E. McKenney
2019-07-24  7:58                                                       ` Byungchul Park
2019-07-24  7:59                                                     ` Byungchul Park
2019-07-12 13:01                     ` Paul E. McKenney
2019-07-12 13:40                       ` Joel Fernandes
2019-07-12  6:00                 ` Byungchul Park
2019-07-12  5:52               ` Byungchul Park
2019-07-12  5:48             ` Byungchul Park
2019-07-13  9:08               ` Byungchul Park

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