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SubjectRe: [PATCH] sched: Optimize branch hint in context_switch()
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.
>

These should be documented then to avoid patches removing them:

#define slowpath(x) unlikely(x)

if (slowpath(condition))
expensive_operation();

--
error compiling committee.c: too many arguments to function



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