lkml.org 
[lkml]   [2009]   [Jan]   [15]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [patch] add optimized generic percpu accessors
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


\
 
 \ /
  Last update: 2009-01-15 11:29    [from the cache]
©2003-2011 Jasper Spaans. Advertise on this site