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Date:	Sat, 13 Dec 2014 04:08:36 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Sasha Levin <sasha.levin@...cle.com>
Cc:	David Lang <david@...g.hm>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Dave Jones <davej@...hat.com>, Chris Mason <clm@...com>,
	Mike Galbraith <umgwanakikbuti@...il.com>,
	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Dâniel Fraga <fragabr@...il.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: frequent lockups in 3.18rc4

On Fri, Dec 12, 2014 at 04:58:07PM -0800, Paul E. McKenney wrote:
> On Fri, Dec 12, 2014 at 04:23:56PM -0500, Sasha Levin wrote:
> > On 12/12/2014 03:34 PM, Paul E. McKenney wrote:
> > > On Fri, Dec 12, 2014 at 11:58:50AM -0800, David Lang wrote:
> > >> > On Fri, 12 Dec 2014, Linus Torvalds wrote:
> > >> > 
> > >>> > >I'm also not sure if the bug ever happens with preemption disabled.
> > >>> > >Sasha, was that you who reported that you cannot reproduce it without
> > >>> > >preemption? It strikes me that there's a race condition in
> > >>> > >__cond_resched() wrt preemption, for example: we do
> > >>> > >
> > >>> > >       __preempt_count_add(PREEMPT_ACTIVE);
> > >>> > >       __schedule();
> > >>> > >       __preempt_count_sub(PREEMPT_ACTIVE);
> > >>> > >
> > >>> > >and in between the __schedule() and __preempt_count_sub(), if an
> > >>> > >interrupt comes in and wakes up some important process, it won't
> > >>> > >reschedule (because preemption is active), but then we enable
> > >>> > >preemption again and don't check whether we should reschedule (again),
> > >>> > >and we just go on our merry ways.
> > >>> > >
> > >>> > >Now, I don't see how that could really matter for a long time -
> > >>> > >returning to user space will check need_resched, and sleeping will
> > >>> > >obviously force a reschedule anyway, so these kinds of races should at
> > >>> > >most delay things by just a tiny amount,
> > >> > 
> > >> > If the machine has NOHZ and has a cpu bound userspace task, it could
> > >> > take quite a while before userspace would trigger a reschedule (at
> > >> > least if I've understood the comments on this thread properly)
> > > Dave, Sasha, if you guys are running CONFIG_NO_HZ_FULL=y and
> > > CONFIG_NO_HZ_FULL_ALL=y, please let me know.  I am currently assuming
> > > that none of your CPUs are in NO_HZ_FULL mode.  If this assumption is
> > > incorrect, there are some other pieces of RCU that I should be taking
> > > a hard look at.
> > 
> > This is my no_hz related config:
> > 
> > $ grep NO_HZ .config
> > CONFIG_NO_HZ_COMMON=y
> > # CONFIG_NO_HZ_IDLE is not set
> > CONFIG_NO_HZ_FULL=y
> > CONFIG_NO_HZ_FULL_ALL=y
> > CONFIG_NO_HZ_FULL_SYSIDLE=y
> > CONFIG_NO_HZ_FULL_SYSIDLE_SMALL=8
> > CONFIG_NO_HZ=y
> > CONFIG_RCU_FAST_NO_HZ=y
> > 
> > And from dmesg:
> > 
> > [    0.000000] Preemptible hierarchical RCU implementation.
> > [    0.000000]  RCU debugfs-based tracing is enabled.
> > [    0.000000]  Hierarchical RCU autobalancing is disabled.
> > [    0.000000]  RCU dyntick-idle grace-period acceleration is enabled.
> > [    0.000000]  Additional per-CPU info printed with stalls.
> > [    0.000000]  RCU restricting CPUs from NR_CPUS=8192 to nr_cpu_ids=28.
> > [    0.000000]  RCU kthread priority: 1.
> > [    0.000000] RCU: Adjusting geometry for rcu_fanout_leaf=16, nr_cpu_ids=28
> > [    0.000000] NR_IRQS:524544 nr_irqs:648 16
> > [    0.000000] NO_HZ: Clearing 0 from nohz_full range for timekeeping
> > [    0.000000] NO_HZ: Full dynticks CPUs: 1-27.
> > [    0.000000]  Offload RCU callbacks from CPUs: 1-27.
> 
> Thank you, Sasha.  Looks like I have a few more places to take a hard
> look at, then!

And one effect of CONFIG_NO_HZ_FULL=y and CONFIG_NO_HZ_FULL_ALL=y is
that all the grace-period kthread are pinned to CPU 0.  In addition,
all of CPUs 1-27 are offloaded, and all of the resulting rcuo kthreads
(which invoke RCU callbacks) are also pinned to CPU 0.  If you are then
running a heavy in-kernel workload that generates lots of callbacks, it
is easy to imagine that CPU 0 might be getting overloaded.  After all,
this combination of Kconfig parameters was designed for HPC and real-time
workloads that spend most of their time in userspace.

If you are allowing your workload to run on CPU 0, it would be very
interesting to see what happens if you restrict your workload to run on
CPUs 1-27.

Alternatively, your could boot with nohz_full=2-27 (or maybe even
nohz_full=4-27).  This will override CONFIG_NO_HZ_FULL_ALL=y and will
provide two (or four with 4-27) housekeeping CPUs that are available to
run things like RCU grace-period kthreads and RCU callback processing.
This might allow RCU to get the CPU bandwidth it needs despite
competition from your workload.

							Thanx, Paul

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