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
>
next prev parent 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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