Re: [PATCH] Fix dirty page accounting in redirty_page_for_writepage()

[Ingo Molnar <mingo@elte.hu>]

* Mathieu Desnoyers <mathieu.desnoyers@polymtl.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