From: Byungchul Park <byungchul.park@lge.com>
To: "Paul E. McKenney" <paulmck@linux.ibm.com>
Cc: Joel Fernandes <joel@joelfernandes.org>,
Byungchul Park <max.byungchul.park@gmail.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 09:54:03 +0900 [thread overview]
Message-ID: <20190719005403.GB28226@X58A-UD3R> (raw)
In-Reply-To: <20190718213419.GV14271@linux.ibm.com>
On Thu, Jul 18, 2019 at 02:34:19PM -0700, Paul E. McKenney wrote:
> On Thu, Jul 18, 2019 at 12:14:22PM -0400, Joel Fernandes wrote:
> > Trimming the list a bit to keep my noise level low,
> >
> > On Sat, Jul 13, 2019 at 1:41 PM Paul E. McKenney <paulmck@linux.ibm.com> wrote:
> > [snip]
> > > > It still feels like you guys are hyperfocusing on this one particular
> > > > > knob. I instead need you to look at the interrelating knobs as a group.
> > > >
> > > > Thanks for the hints, we'll do that.
> > > >
> > > > > On the debugging side, suppose someone gives you an RCU bug report.
> > > > > What information will you need? How can you best get that information
> > > > > without excessive numbers of over-and-back interactions with the guy
> > > > > reporting the bug? As part of this last question, what information is
> > > > > normally supplied with the bug? Alternatively, what information are
> > > > > bug reporters normally expected to provide when asked?
> > > >
> > > > I suppose I could dig out some of our Android bug reports of the past where
> > > > there were RCU issues but if there's any fires you are currently fighting do
> > > > send it our way as debugging homework ;-)
> > >
> > > Suppose that you were getting RCU CPU stall
> > > warnings featuring multi_cpu_stop() called from cpu_stopper_thread().
> > > Of course, this really means that some other CPU/task is holding up
> > > multi_cpu_stop() without also blocking the current grace period.
> > >
> >
> > So I took a shot at this trying to learn how CPU stoppers work in
> > relation to this problem.
> >
> > I am assuming here say CPU X has entered MULTI_STOP_DISABLE_IRQ state
> > in multi_cpu_stop() but another CPU Y has not yet entered this state.
> > So CPU X is stalling RCU but it is really because of CPU Y. Now in the
> > problem statement, you mentioned CPU Y is not holding up the grace
> > period, which means Y doesn't have any of IRQ, BH or preemption
> > disabled ; but is still somehow stalling RCU indirectly by troubling
> > X.
> >
> > This can only happen if :
> > - CPU Y has a thread executing on it that is higher priority than CPU
> > X's stopper thread which prevents it from getting scheduled. - but the
> > CPU stopper thread (migration/..) is highest priority RT so this would
> > be some kind of an odd scheduler bug.
> > - 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.
>
> - CPU Y might have already passed through its quiescent state for
> the current grace period, then disabled IRQs indefinitely.
Or for a longer time than the period that rcu considers as a stall. Or
preemption disabled for that long time. Or the stopper on Y even has yet
to be woken up inside scheduler because of any reasons but maybe locks.
> Now, CPU Y would block a later grace period, but CPU X is
> preventing the current grace period from ending, so no such
> later grace period can start.
>
> > Did I get that right? Would be exciting to run the rcutorture test
> > once Paul has it available to reproduce this problem.
>
> Working on it! Slow, I know!
>
> Thanx, Paul
next prev parent reply other threads:[~2019-07-19 0:55 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 [this message]
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
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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