lkml.org 
[lkml]   [2011]   [Aug]   [5]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [RFT][patch 17/18] sched: use jump labels to reduce overhead when bandwidth control is inactive
On Fri, Aug 05, 2011 at 08:11:15AM -0700, Richard Henderson wrote:
> On 08/05/2011 01:30 AM, Peter Zijlstra wrote:
> > OK, so _WHY_ does that make a difference and will a next version of
> > gnu-binutils not mess that up?
>
> The Why is micro-architectual, and I can't answer that.

In tracking this down, I eventually found that just having the
jump_label.o file compiled into the kernel, but not actually using the
static_branch(), or 'asm goto' anywhere, led to a performance hit.
Thus, the compiler or the 'asm goto' itself wasn't actually causing any
degradation.

Since the jump_label.o file is only slow-path code, it can be moved away
from core or heavily called kernel routines. I suspect this is probably
an icache issue, but I can't say for sure.

Thanks,

-Jason

>
> But ld will never re-order the files as given on the command-line.
> There are too many functions and tables that are constructed
> piece-wise from input sections; re-ordering them would change
> the semantics of the program.
>
>
> r~


\
 
 \ /
  Last update: 2011-08-05 17:27    [W:0.070 / U:0.216 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site