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

[Avi Kivity]

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

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/