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

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Message-ID: <alpine.LSU.2.00.1103182037340.18458@sister.anvils>
Date:	Fri, 18 Mar 2011 20:58:38 -0700 (PDT)
From:	Hugh Dickins <hughd@...gle.com>
To:	Christoph Lameter <cl@...ux.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 Thu, 17 Mar 2011, Christoph Lameter wrote:
> 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?

CONFIG_GENERIC_LOCKBREAK adds an unsigned int break_lock after the
int-sized arch_spinlock_t: which would make no difference on 64-bit
anyway (the two ints fitting into one long), and makes no difference
on 32-bit because we have put
	struct {
		unsigned long private;
		struct address_space *mapping;
	};
into the union with spinlock_t ptl - the arch_spinlock_t then
overlays private and the break_lock overlays mapping.

I'd much rather have had simple elements in that union, but it's
precisely because of 32-bit CONFIG_GENERIC_LOCKBREAK that we need
that structure in there.

(It is important to the KSM assumption about page->mapping that
what goes into break_lock is either 0 or 1: in neither case could
page->mapping look like a kmem address + 3.)

Hugh
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