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

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Message-ID: <20090501204355.GB20280@Krystal>
Date:	Fri, 1 May 2009 16:43:55 -0400
From:	Mathieu Desnoyers <compudj@...stal.dyndns.org>
To:	Christoph Lameter <cl@...ux.com>
Cc:	Nick Piggin <nickpiggin@...oo.com.au>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Yuriy Lalym <ylalym@...il.com>, ltt-dev@...ts.casi.polymtl.ca,
	Tejun Heo <tj@...nel.org>, Ingo Molnar <mingo@...e.hu>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [ltt-dev] [PATCH] Fix dirty page accounting in
	redirty_page_for_writepage()

* Christoph Lameter (cl@...ux.com) wrote:
> On Fri, 1 May 2009, Mathieu Desnoyers wrote:
> 
> > So, back to my original point : do you agree on the usefulness of
> > separating fallback irq-disabling from the per-cpu atomic construct ?
> 
> No. Percpu operations are used for statistics and are like atomic
> operations. Aggregation of these leads to a can of worms that we better
> leave unopened.
> 
> > x86 would map :
> >
> > percpu_irqsave/restore to "nothing".
> > percpu_add_return_irq to xadd instruction. It is irq-safe by design.
> >
> > Other architectures (fallback) would map
> >
> > percpu_irqsave/restore to local_irq_save/restore.
> > percpu_add_return_irq to var += value; return var;
> 
> Shudder.... We have explored those types of macros before (while doing
> fastpath optimization for SLUB) and it significant increases the
> complexity. People may add additional instructions in between and now
> interrupts off could be on or off depending on the architecture. Sometimes
> percpu_irqsave does nothing. Very difficult to ensure that the usage is
> correct.
> 
> And we have barely any usage case for such macros.
> 

Then do you have a better idea on how to deal with
__inc_zone_state/inc_zone_state without duplicating the code and without
adding useless local_irq_save/restore on x86 ?

Mathieu

-- 
Mathieu Desnoyers
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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