Messages in this thread | | | Date | Thu, 6 Aug 2020 16:45:33 +0200 | From | Ingo Molnar <> | Subject | Re: [PATCH] sched/core: add unlikely in group_has_capacity() |
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* Qi Zheng <arch0.zheng@gmail.com> 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@gmail.com> > --- > 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
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