linux-kernel - Re: [PATCH] Fix dirty page accounting in redirty_page_for

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Message-ID: <alpine.DEB.1.10.0904301209140.4028@qirst.com>
Date:	Thu, 30 Apr 2009 12:11:26 -0400 (EDT)
From:	Christoph Lameter <cl@...ux.com>
To:	Ingo Molnar <mingo@...e.hu>
cc:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Nick Piggin <nickpiggin@...oo.com.au>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>, thomas.pi@...or.dea,
	Yuriy Lalym <ylalym@...il.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	ltt-dev@...ts.casi.polymtl.ca, Tejun Heo <tj@...nel.org>
Subject: Re: [PATCH] Fix dirty page accounting in
 redirty_page_for_writepage()

On Thu, 30 Apr 2009, Ingo Molnar wrote:

> > may not care too much about a event counter missing a beat once in
> > a while for platforms not supporting atomic per cpu ops. I know
> > this affects IA64. The cost of an atomic operations for an event
> > counter update (which would have avoided the potential of a
> > concurrent update) was not justifiable.
>
> when you say "atomics", do you mean the classic meaning of atomics?
> Because there are no classic atomics involved. This is the
> before/after disassembly from Eric's commit 4e69489a0:

The fallback for IA64 would be to use full (classic) atomic operations
(fetchadd) instead of fast atomic vs. interrupt as available on x86

> c0436275:   64 83 05 20 5f 6a c0    addl   $0x1,%fs:0xc06a5f20
>
> There's no atomic instructions at all - the counters here are only
> accessed locally. They are local-irq-atomic, but not
> cacheline-atomic.

Right but that is not available on IA64. So one must choose between
manually disabling interrupts and then increment the counter (long code
sequence) and a classic atomic operation for the fallback.
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