Re: [PATCH] sched/core: add unlikely in group_has_capacity()

[Qi Zheng]

Yeah, because of the following two points, I also think
the probability is 0%:
a) the sd is protected by rcu lock, and load_balance()
   func is between rcu_read_lock() and rcu_read_unlock().
b) the sgs is a local variable.
	
So in the group_classify(), the env->sd->imbalance_pct and
the sgs will not be changed. May I remove the duplicate check
from group_has_capacity() and resubmit a patch?

Yours,
Qi Zheng

On 2020/8/6 下午10:45, Ingo Molnar wrote:
> * Qi Zheng <arch0.zheng@xxxxxxxxx> wrote:
>
> > 1. The group_has_capacity() function is only called in
> >     group_classify().
> > 2. Before calling the group_has_capacity() function,
> >     group_is_overloaded() will first judge the following
> >     formula, if it holds, the group_classify() will directly
> >     return the group_overloaded.
> >
> > 	(sgs->group_capacity * imbalance_pct) <
> >                          (sgs->group_runnable * 100)
> >
> > Therefore, when the group_has_capacity() is called, the
> > probability that the above formalu holds is very small. Hint
> > compilers about that.
> >
> > Signed-off-by: Qi Zheng <arch0.zheng@xxxxxxxxx>
> > ---
> >   kernel/sched/fair.c | 4 ++--
> >   1 file changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index 2ba8f230feb9..9074fd5e23b2 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -8234,8 +8234,8 @@ group_has_capacity(unsigned int imbalance_pct, struct sg_lb_stats *sgs)
> >   	if (sgs->sum_nr_running < sgs->group_weight)
> >   		return true;
> >   
> > -	if ((sgs->group_capacity * imbalance_pct) <
> > -			(sgs->group_runnable * 100))
> > +	if (unlikely((sgs->group_capacity * imbalance_pct) <
> > +			(sgs->group_runnable * 100)))
> >   		return false;
>
> Isn't the probability that this second check will match around 0%?
>
> I.e. wouldn't the right fix be to remove the duplicate check from
> group_has_capacity(), because it's already been checked in
> group_classify()? Maybe while leaving a comment in place?
>
> Thanks,
>
> 	Ingo