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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
