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

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Message-ID: <20090430062140.GA9559@elte.hu>
Date:	Thu, 30 Apr 2009 08:21:40 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
Cc:	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, Christoph Lameter <cl@...ux.com>
Subject: Re: [PATCH] Fix dirty page accounting in
	redirty_page_for_writepage()


* Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca> wrote:

> And thanks for the review! This excercise only convinced me that 
> the kernel memory accounting works as expected. All this gave me 
> the chance to have a good look at the memory accounting code. We 
> could probably benefit of Christoph Lameter's cpu ops (using 
> segment registers to address per-cpu variables with atomic 
> inc/dec) in there. Or at least removing interrupt disabling by 
> using preempt disable and local_t variables for the per-cpu 
> counters could bring some benefit.

Note, optimized per cpu ops are already implemented upstream, by 
Tejun Heo's percpu patches in .30:

 #define percpu_read(var)	percpu_from_op("mov", per_cpu__##var)
 #define percpu_write(var, val)	percpu_to_op("mov", per_cpu__##var, val)
 #define percpu_add(var, val)	percpu_to_op("add", per_cpu__##var, val)
 #define percpu_sub(var, val)	percpu_to_op("sub", per_cpu__##var, val)
 #define percpu_and(var, val)	percpu_to_op("and", per_cpu__##var, val)
 #define percpu_or(var, val)	percpu_to_op("or", per_cpu__##var, val)
 #define percpu_xor(var, val)	percpu_to_op("xor", per_cpu__##var, val)

See:

  6dbde35: percpu: add optimized generic percpu accessors

>From the changelog:

    [...]
    The advantage is that for example to read a local percpu variable,
    instead of this sequence:
    
     return __get_cpu_var(var);
    
     ffffffff8102ca2b:  48 8b 14 fd 80 09 74    mov    -0x7e8bf680(,%rdi,8),%rdx
     ffffffff8102ca32:  81
     ffffffff8102ca33:  48 c7 c0 d8 59 00 00    mov    $0x59d8,%rax
     ffffffff8102ca3a:  48 8b 04 10             mov    (%rax,%rdx,1),%rax
    
    We can get a single instruction by using the optimized variants:
    
     return percpu_read(var);
    
     ffffffff8102ca3f:  65 48 8b 05 91 8f fd    mov    %gs:0x7efd8f91(%rip),%rax
    [...]

So if you want to make use of it, percpu_add()/percpu_sub() would be 
the place to start.

	Ingo
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