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SubjectRe: [tip:core/rcu] Revert "rcu: Decrease memory-barrier usage based on semi-formal proof"
On 05/24/2011 02:23 PM, Yinghai Lu wrote:
> On 05/23/2011 06:35 PM, Paul E. McKenney wrote:
>> On Mon, May 23, 2011 at 06:26:23PM -0700, Yinghai Lu wrote:
>>> On 05/23/2011 06:18 PM, Paul E. McKenney wrote:
>>>
>>>> OK, so it looks like I need to get this out of the way in order to track
>>>> down the delays. Or does reverting PeterZ's patch get you a stable
>>>> system, but with the longish delays in memory_dev_init()? If the latter,
>>>> it might be more productive to handle the two problems separately.
>>>>
>>>> For whatever it is worth, I do see about 5% increase in grace-period
>>>> duration when switching to kthreads. This is acceptable -- your
>>>> 30x increase clearly is completely unacceptable and must be fixed.
>>>> Other than that, the main thing that affects grace period duration is
>>>> the setting of CONFIG_HZ -- the smaller the HZ value, the longer the
>>>> grace-period duration.
>>>
>>> for my 1024g system when memory hotadd is enabled in kernel config:
>>> 1. current linus tree + tip tree: memory_dev_init will take about 100s.
>>> 2. current linus tree + tip tree + your tree - Peterz patch:
>>> a. on fedora 14 gcc: will cost about 4s: like old times
>>> b. on opensuse 11.3 gcc: will cost about 10s.
>>
>> So some patch in my tree that is not yet in tip makes things better?
>>
>> If so, could you please see which one? Maybe that would give me a hint
>> that could make things better on opensuse 11.3 as well.
>
> today's tip:
>
> [ 31.795597] cpu_dev_init done
> [ 40.930202] memory_dev_init done
>

another boot from tip got:

[ 35.211927] cpu_dev_init done
[ 136.053698] memory_dev_init done

wonder if you can have clean revert for

commit a26ac2455ffcf3be5c6ef92bc6df7182700f2114
> Author: Paul E. McKenney <paul.mckenney@linaro.org>
> Date: Wed Jan 12 14:10:23 2011 -0800
>
> rcu: move TREE_RCU from softirq to kthread
>
> If RCU priority boosting is to be meaningful, callback invocation must
> be boosted in addition to preempted RCU readers. Otherwise, in presence
> of CPU real-time threads, the grace period ends, but the callbacks don't
> get invoked. If the callbacks don't get invoked, the associated memory
> doesn't get freed, so the system is still subject to OOM.
>
> But it is not reasonable to priority-boost RCU_SOFTIRQ, so this commit
> moves the callback invocations to a kthread, which can be boosted easily.
>
> Also add comments and properly synchronized all accesses to
> rcu_cpu_kthread_task, as suggested by Lai Jiangshan.
>
> Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> Reviewed-by: Josh Triplett <josh@joshtriplett.org>

Thanks

Yinghai Lu


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