linux-kernel - Re: [PATCH] sched: Optimize branch hint in context

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Date:	Sun, 29 Nov 2009 17:12:37 +0200
From:	Avi Kivity <avi@...hat.com>
To:	Tim Blechmann <tim@...ngt.org>
CC:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	linux-kernel@...r.kernel.org,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] sched: Optimize branch hint in context_switch()

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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