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

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Message-ID: <4B129277.5050000@redhat.com>
Date:	Sun, 29 Nov 2009 17:25:43 +0200
From:	Avi Kivity <avi@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	Tim Blechmann <tim@...ngt.org>, linux-kernel@...r.kernel.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 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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