linux-kernel - Re: [tip:core/rcu] Revert "rcu: Decrease memory-barrier usage based on semi-formal proof"

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Date:	Tue, 24 May 2011 17:10:11 -0700
From:	Yinghai Lu <yinghai@...nel.org>
To:	paulmck@...ux.vnet.ibm.com
CC:	linux-kernel@...r.kernel.org, mingo@...hat.com, hpa@...or.com,
	tglx@...utronix.de, mingo@...e.hu
Subject: Re: [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@...aro.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@...aro.org>
>     Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
>     Reviewed-by: Josh Triplett <josh@...htriplett.org>

Thanks

Yinghai Lu
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