Re: [PATCH] sched: Optimize branch hint in context_switch()

[Avi Kivity]

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.

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

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