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

Ingo Molnar
2009-04-30T06:22:44Z

* 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