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

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Message-ID: <4DDD7F96.3090408@kernel.org>
Date:	Wed, 25 May 2011 15:15:50 -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 09:52 PM, Paul E. McKenney wrote:
> On Tue, May 24, 2011 at 05:10:11PM -0700, Yinghai Lu wrote:
>> 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>
> 
> There is a new branch yinghai.2011.05.24a on:
> 
> git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-2.6-rcu.git
> 
> Or will be as soon as kernel.org updates its mirrors.
> 
> I am not sure I could call this "clean", but it does revert that commit
> and 11 of the subsequent commits that depend on it.  It does build,
> and I will test it once my currently running tests complete.

yes, with those revert, there is no delay in 10 times booting.

Thanks

Yinghai
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