netdev - Re: [PATCH 2/4] slub,rcu: don't assume the size of struct rcu

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Date:	Thu, 17 Mar 2011 10:16:01 -0500 (CDT)
From:	Christoph Lameter <cl@...ux.com>
To:	Hugh Dickins <hughd@...gle.com>
cc:	Pekka Enberg <penberg@...nel.org>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	Ingo Molnar <mingo@...e.hu>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Eric Dumazet <eric.dumazet@...il.com>,
	"David S. Miller" <davem@...emloft.net>,
	Matt Mackall <mpm@...enic.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: [PATCH 2/4] slub,rcu: don't assume the size of struct rcu_head

On Sun, 6 Mar 2011, Hugh Dickins wrote:

> >> That was so for a long time, but I stopped it just over a year ago
> >> with commit a70caa8ba48f21f46d3b4e71b6b8d14080bbd57a, stop ptlock
> >> enlarging struct page.
> >
> > Strange. I just played around with in in January and the page struct size
> > changes when I build kernels with full debugging. I have some
> > cmpxchg_double patches here that depend on certain alignment in the page
> > struct. Debugging causes all that stuff to get out of whack so that I had
> > to do some special patches to make sure fields following the spinlock are
> > properly aligned when the sizes change.
>
> That puzzles me, it's not my experience and I don't have an
> explanation: do you have time to investigate?
>
> Uh oh, you're going to tell me you're working on an out-of-tree
> architecture with a million cpus ;)  In that case, yes, I'm afraid
> I'll have to update the SPLIT_PTLOCK_CPUS defaulting (for a million -
> 1 even).

No I am not working on any out of tree structure. Just regular dual socket
server boxes with 24 processors (which is a normal business machine
configuration these days).

But then there is also CONFIG_GENERIC_LOCKBREAK. That does not affect
things?

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