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

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Date:	Mon, 23 May 2011 18:26:23 -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 05/23/2011 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.

Thanks

Yinghai Lu

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