Messages in this thread | | | Date | Fri, 5 Aug 2011 11:24:12 -0400 | From | Jason Baron <> | Subject | Re: [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~
| |