lkml.org 
[lkml]   [2009]   [Nov]   [29]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
From
SubjectRe: [PATCH] sched: Optimize branch hint in context_switch()
Date
On 11/29/2009 04:25 PM, Avi Kivity wrote:
> On 11/29/2009 05:20 PM, Peter Zijlstra wrote:
>> On Sun, 2009-11-29 at 17:12 +0200, Avi Kivity wrote:
>>
>>> On 11/29/2009 02:01 PM, Tim Blechmann wrote:
>>>
>>>> Branch hint profiling on my nehalem machine showed 88%
>>>> incorrect branch hints:
>>>>
>>>> 42017484 326957902 88 context_switch sched.c 3043
>>>> 42038493 326953687 88 context_switch sched.c 3050
>>>>
>>>> @@ -3040,14 +3040,14 @@ context_switch(struct rq *rq, struct task_struct *prev,
>>>> */
>>>> arch_start_context_switch(prev);
>>>>
>>>> - if (likely(!mm)) {
>>>> + if (unlikely(!mm)) {
>>>> next->active_mm = oldmm;
>>>> atomic_inc(&oldmm->mm_count);
>>>> enter_lazy_tlb(oldmm, next);
>>>> } else
>>>> switch_mm(oldmm, mm, next);
>>>>
>>>> - if (likely(!prev->mm)) {
>>>> + if (unlikely(!prev->mm)) {
>>>> prev->active_mm = NULL;
>>>> rq->prev_mm = oldmm;
>>>> }
>>>>
>>>>
>>> I don't think either the original or the patch is correct. Whether or
>>> not a task has an mm is entirely workload dependent, we shouldn't be
>>> giving hints here.
>>>
>> There are reasons to still use branch hints, for example if the unlikely
>> branch is very expensive anyway and it pays to have the likely branch be
>> ever so slightly less expensive.
>>
>> Now I don't think that applies here, but there are cases where such code
>> generation issues are the main motivator not the actual usage patterns.

would be nice, if you commit a patch, removing this hint

> These should be documented then to avoid patches removing them:
>
> #define slowpath(x) unlikely(x)
>
> if (slowpath(condition))
> expensive_operation();

this would definitely improve the expressive power ...

thnx, tim

--
tim@klingt.org
http://tim.klingt.org

Only very good and very bad programmers use goto in C

[unhandled content-type:application/pgp-signature]
\
 
 \ /
  Last update: 2009-11-29 17:05    [W:0.038 / U:1.228 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site