Re: [patch] add optimized generic percpu accessors

[Tejun Heo]

Hello, Ingo.

Ingo Molnar wrote:
> Tejun, could you please also add the patch below to your lineup too?

Sure thing.

> It is an optimization and a cleanup, and adds the following new generic 
> percpu methods:
> 
>   percpu_read()
>   percpu_write()
>   percpu_add()
>   percpu_sub()
>   percpu_or() 
>   percpu_xor()
> 
> and implements support for them on x86. (other architectures will fall 
> back to a default implementation)
> 
> 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
> 
> I also cleaned up the x86-specific APIs and made the x86 code use these 
> new generic percpu primitives.
> 
> It looks quite hard to convince the compiler to generate the optimized 
> single-instruction sequence for us out of __get_cpu_var(var) - or can you 
> perhaps see a way to do it?

Yeah, I thought about that too but couldn't think of a way to persuade
the compiler because the compiler doesn't know how to access the
address.  I'll play with it a bit more but the clumsy percpu_*()
accessors probably might be the only way.  :-(

Thanks.

-- 
tejun
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/