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