Re: frequent lockups in 3.18rc4

[Sasha Levin]

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.

Thanks,
Sasha
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