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

From: Avi Kivity
Date: Sun Nov 29 2009 - 10:13:21 EST


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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