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

[Qi Zheng <arch0.zheng@gmail.com>]

On 2020/8/7 上午10:47, Qi Zheng wrote:
> 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
>>

Hi,

As Valentin and I discussed in the patch below, simply removing the
check may not be completely harmless.

	[PATCH]sched/fair: Remove the duplicate check from
					group_has_capacity() :
	-	if ((sgs->group_capacity * imbalance_pct) <
	-			(sgs->group_runnable * 100))
	-		return false;


If sum_nr_running < group_weight, we won't evaluate it.
If sum_nr_running > group_weight, we either won't call into
   group_has_capacity() or we'll have checked it already in
   group_overloaded().
But in the case of sum_nr_running == group_weight, we can
run to this check.

Although I also think it is unlikely to cause the significant
capacity pressure at the == case, but I'm not sure whether there
are some special scenarios. such as some cpus in sg->cpumask are
no longer active, or other scenarios?

So adding the unlikely() in group_has_capacity() may be the safest
way.

Add Valentin Schneider <valentin.schneider@arm.com>.

Yours,
Qi Zheng