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Date:   Thu, 7 Dec 2017 09:29:06 -0800
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Frederic Weisbecker <frederic@...nel.org>
Cc:     linux-kernel@...r.kernel.org, xiaolong.ye@...el.com,
        tglx@...utronix.de, cmetcalf@...lanox.com, cl@...ux.com,
        torvalds@...ux-foundation.org, lcapitulino@...hat.com,
        efault@....de, peterz@...radead.org, riel@...hat.com,
        kernellwp@...il.com, mingo@...nel.org, john.stultz@...aro.org
Subject: Re: [PATCH] sched/isolation: Make NO_HZ_FULL select CPU_ISOLATION

On Thu, Dec 07, 2017 at 05:14:54PM +0100, Frederic Weisbecker wrote:
> 2017-12-04 18:16 UTC+01:00, Paul E. McKenney <paulmck@...ux.vnet.ibm.com>:
> > On Mon, Dec 04, 2017 at 04:53:15PM +0100, Frederic Weisbecker wrote:
> >> 2017-12-02 20:24 UTC+01:00, Paul E. McKenney
> >> I would prefer to keep it. It's useful for automated boot testing
> >> based on configs such as 0-day or -tip test machines. But I'm likely
> >> to migrate it to isolcpus implementation. Maybe something along the
> >> lines of CONFIG_CPU_ISOLATION_ALL.
> >
> > How about instead allowing something like "nohz_full=1-" specify that
> > all CPUs other than CPU 0 should be nohz_full CPUs?  That would shrink
> > the code by eliminating CONFIG_NO_HZ_FULL_ALL while still allowing
> > easy automation of that particular scenario.
> >
> > (Right now, the boot code complains about "nohz_full=1-", which means
> > that whatever is generating the boot parameters needs to know how many
> > CPUs there really are, which as you say can be a pain.)
> 
> Yes but automated boot testing is rather based on configs than boot
> options. It's much easier. I think that's how Wu Fengguang lab works,
> and -tip automated tests as well.

So you have gotten bug reports from them?  Because I see splats from
rcutorture testing rather frequently.  This thing is in no way a subtle
low-probability bug.  ;-)

> >> >> Did you have any nohz_full= or isolcpus= boot options?
> >> >
> >> > Replacing CONFIG_NO_HZ_FULL_ALL=y with nohz_full=1-7 works, that
> >> > is CONFIG_NO_HZ_FULL=y, CONFIG_NO_HZ_FULL_ALL=n, and nohz_full=1-7
> >> > on an eight-CPU test.
> >> >
> >> > But it is relatively easy to test.  Running the rcutorture TREE04
> >> > scenario on a four-socket x86 gets me RCU CPU stall warnings within
> >> > a few minutes more than half the time.  ;-)
> >>
> >> Indeed I managed to trigger something. If it's the same thing I should
> >> be able to track down the root cause.
> >>
> >> [  123.907557] ??? Writer stall state RTWS_STUTTER(8) g160 c160 f0x0
> >> ->state 0x1 cpu 2
> >> [  123.915184] rcu_torture_wri S    0   111      2 0x80080000
> >> [  123.920673] Call Trace:
> >> [  123.923096]  ? __schedule+0x2bf/0xbb0
> >> [  123.926715]  ? _raw_spin_unlock_irqrestore+0x59/0x70
> >> [  123.931657]  schedule+0x3c/0x90
> >> [  123.934777]  schedule_timeout+0x1e1/0x560
> >
> > It might well be the same thing, as this schedule_timeout() does look
> > familiar.  I have some diagnostic patches in -rcu, please see below
> > for the overall effect.
> 
> I fear I can hit that even without any nohz_full CPU as well.

Indeed, I do hit that with my TREE01 scenario, which does not set
CONFIG_NO_HZ_FULL.  But it is much less frequent.  The good news is that
I have finally figured out a way to extract information from this thing
without suppressing it.  At the moment it appears to be a rather strange
deadlock involving CPU hotplug, timers, and RCU.

But that is a completely different bug from the ones for which I have
the two patches in my tree.

Anyway, I will keep those two patches because I cannot have the
corresponding bugs possibly hiding RCU bugs in my testing.  If you
put some other fix in place, I will drop those two patches in favor of
your fix.

							Thanx, Paul

> >> [  123.938785]  ? __next_timer_interrupt+0xd0/0xd0
> >> [  123.943276]  stutter_wait+0xc5/0xe0
> >> [  123.946738]  rcu_torture_writer+0x1ae/0x730
> >> [  123.950912]  ? rcu_torture_pipe_update+0xf0/0xf0
> >> [  123.955491]  kthread+0x15f/0x1a0
> >> [  123.958702]  ? kthread_unpark+0x60/0x60
> >> [  123.962523]  ret_from_fork+0x24/0x30
> >> [  123.966091] rcu_preempt: wait state: 1 ->state: 0x402
> >> [  123.971112] rcu_sched: wait state: 1 ->state: 0x402
> >> [  123.975953] rcu_bh: wait state: 1 ->state: 0x402
> >
> > ------------------------------------------------------------------------
> >
> > diff --git a/kernel/time/timer.c b/kernel/time/timer.c
> > index ffebcf878fba..23af27461d8c 100644
> > --- a/kernel/time/timer.c
> > +++ b/kernel/time/timer.c
> > @@ -1755,8 +1755,13 @@ static void process_timeout(struct timer_list *t)
> >   */
> >  signed long __sched schedule_timeout(signed long timeout)
> >  {
> > +	struct timer_base *base;
> >  	struct process_timer timer;
> >  	unsigned long expire;
> > +	unsigned long flags;
> > +	unsigned long i;
> > +	unsigned int idx, idx_now;
> > +	unsigned long j;
> >
> >  	switch (timeout)
> >  	{
> > @@ -1793,6 +1798,17 @@ signed long __sched schedule_timeout(signed long
> > timeout)
> >  	timer_setup_on_stack(&timer.timer, process_timeout, 0);
> >  	__mod_timer(&timer.timer, expire, 0);
> >  	schedule();
> > +	j = jiffies;
> > +	if (timeout < 5 && time_after(j, expire + 8 * HZ) &&
> > timer_pending(&timer.timer)) {
> > +		base = lock_timer_base(&timer.timer, &flags);
> > +		idx = timer_get_idx(&timer.timer);
> > +		idx_now = calc_wheel_index(j, base->clk);
> > +		raw_spin_unlock_irqrestore(&base->lock, flags);
> > +		pr_info("%s: Waylayed timer base->clk: %#lx jiffies: %#lx
> > base->next_expiry: %#lx timer->flags: %#x timer->expires %#lx idx: %x
> > idx_now: %x base->pending_map ", __func__, base->clk, j, base->next_expiry,
> > timer.timer.flags, timer.timer.expires, idx, idx_now);
> > +		for (i = 0; i < WHEEL_SIZE / sizeof(base->pending_map[0]) / 8; i++)
> > +			pr_cont("%016lx", base->pending_map[i]);
> > +		pr_cont("\n");
> > +	}
> >  	del_singleshot_timer_sync(&timer.timer);
> >
> >  	/* Remove the timer from the object tracker */
> >
> >
> 
> Hmm, that message doesn't seem to trigger :-s
> 
> Thanks.
> 

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