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 21:52:53 -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 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.

							Thanx, Paul
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