Messages in this thread | | | From | Tim Blechmann <> | Subject | Re: [PATCH] sched: Optimize branch hint in context_switch() | Date | Sun, 29 Nov 2009 17:02:51 +0100 |
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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
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