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

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Message-ID:  <4B129B2B.1040309@klingt.org>
Date:	Sun, 29 Nov 2009 17:02:51 +0100
From:	Tim Blechmann <tim@...ngt.org>
To:	linux-kernel@...r.kernel.org
Cc:	Peter Zijlstra <peterz@...radead.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 04:25 PM, Avi Kivity wrote:
> 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.

would be nice, if you commit a patch, removing this hint

> These should be documented then to avoid patches removing them:
> 
>       #define slowpath(x) unlikely(x)
> 
>       if (slowpath(condition))
>             expensive_operation();

this would definitely improve the expressive power ...

thnx, tim

-- 
tim@...ngt.org
http://tim.klingt.org

Only very good and very bad programmers use goto in C


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