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

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Date:	Wed, 25 May 2011 15:34:37 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Yinghai Lu <yinghai@...nel.org>
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 Wed, May 25, 2011 at 03:15:50PM -0700, Yinghai Lu wrote:
> 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.

Unfortunately, there are rcutorture test failures with the revert...

							Thanx, Paul
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